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FEBRUARY, 1948
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AUDIO ENGINEERING
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FEBRUARY, 1948
John H. Potts, Editor
Member of Audit Bureau of Circulation
C.
áD10
G. McProud, Managing Editor
David Saltman, Adv. Prod. Mgr.
Jerome S. Harris, Asst. Circ. Mgr.
Paul
RADIO
A.
Representatives
James C. Galloway
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de Mars
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INEERING
Sanford R. Cowan, Publisher
Young White
Harris á Floyd
297 Swanston St., Melbourne
C. 1, Victoria, Australia
FEBRUARY, 1948
CONTENTS
Vol. 32, No. 2
Editor's Report
4
Letters
6
Broadcasting Studio Pickup Technique -H. M. Gurin
9
Sound Reinforcement in the Hollywood Bowl-M. Rettinger, and Sterling M.
Stevens
Analyzing Intermodulation
15
17
Feedback Preamplifier for Magnetic Pickups-Richard
Two -Way Speaker System, Part
III-C.
Facts About Loudspeakers, Part I
S. Burwen
G. McProud
-0. L. Angevine, Jr., and R. S. Anderson
Cavity Pressure Determination of Hearing Aid Gain
Elements of Ultrasonics S. Young White
I.R.E. 1948 National Convention, Tentative Technical Program
18
21
24
27
28
30
Record Revue:
Classical Recordings --Edward Tafna!! Canby
Popular Recordings- -Bertram Stanleieh
32
32
New Products...
34
Technicana: High- Quality Amplifier, Synchrodyne Receiver, Changes in Standard
Frequency Broadcast
Professional Directory
36
Advertising Index
48
36
COVER ILLUSTRATION
Control room of Studio
1,
KSL, Salt Lake City, Utah.
AUDIO ENGINEERING (title registered U.S. Pat. Off.) is published monthly at 28 Renne Ave.. Pittsfield. Massachusetts, by Radio Magazines. Inc.. J. H. Potts.
President: S. R. Cowan. Sec 'y-Tree. Executive and Editorial Office at 342 Madison Avenue. New York 17. N. Y. Subscription rata-United States. U. S. Poresaioos
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I
AUDIO ENGINEERING FEBRUARY, 1948
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3
EDITOR'S REPORT
NUMBER ONE BOOK
A FEW months ago one of the largest advertising
agencies conducted a survey to determine which of the
eight trade publications serving the broadcasting
field were preferred by broadcast station technical personnel. When the returns were tabulated, three
magazines ran neck and neck, with the others trailing
badly. Audio Engineering, although in existence
but six months, was rated among the top three.
More recently, a manufacturer made a similar
survey. This time, with two more excellent issues
under our belt and with our increased circulation,
Audio Engineering was rated best in the field.
MORE ON
HI -FI
IN THE same mail we received two letters which
point up the wide gulf between the two schools of
thought on high fidelity. From England, a British
engineer writes that we shouldn't call an amplifier
"high- fidelity" unless the harmonic distortion is kept
down to around one -tenth of one per cent and the
frequency response flat to within one db from 20 to
20,000 cycles. The other writer maintained that his
amplifier had to be designed to boost both lows and
highs far above the middle register to give him satisfactory reception. Canby has written in his column
that what we really want is pleasing reception, whether
or not it is high fidelity, and cites the fact that even a
pleasant voice might sound awful if the speaker got
too close to the mike, especially if exactly reproduced.
All this has its parallel, of course, in other fields.
A couple of decades ago, the same controversy arose in
photography. Portrait photographers didn't like high
grade anastigmat lenses because they brought out
every pore and blemish in the skin, details which were
not normally noticed when directly viewed by the
eye. A fad arose for partially corrected lenses, chiffon
diffusers, and other means of softening the details.
In some cases, these expedients did give more pleasing
pictures, but these devices have largely disappeared
with the advent of better photographic materials and
improved techniques in lighting and finishing processes.
Because a reduction in sound power causes a far
greater decrease in the loudness of the lower fre4
quencies (and, to a lesser degree, the higher frequencies) than those in the middle register, due to the
characteristics of the human ear, it has been argued
that some compensation is necessary when reproducing sounds at a lower power level. This is not necessarily true. The sound power developed by a large
orchestra, for example, is far greater than that produced by the average radio. But the orchestra would
normally be spreading this acoustic power over a much
greater area than that covered by a home radio when
operated in a living room. Thus the radio could reproduce in the home orchestra music with much less
sound power, yet give the same degree of loudness as
would be experienced by the listener to the orchestra
at some point in a large auditorium. Therefore no
bass boost is necessary unless the listener operates
the reproducing equipment so that the music is not
as loud as it would be if he were listening to the
orchestra directly in an auditorium.
Engineers who test loudspeakers are often somewhat
amused at the efforts of designers of amplifiers to
make the electrical response flat to within a fraction
of a db over a wide frequency range, because the
speakers to which the amplifiers connect have such
jagged response curves. Actually, if uniform frequency
response were the only consideration, the care would
not be worth the trouble. But in making the frequency
response flat, distortion is also reduced, so that a fine
amplifier does enable better reproduction from the
same speaker than could be secured from a mediocre
design.
WITH OUR AUTHORS
WINSTON WELLS has recovered from his illness
and his next article on the design of electronic organs
will appear in our March issue. C.J. LeBel and Norman
Pickering dropped in to tell how well they are progressing in organizing an audio engineering society.
Had lunch with Howard Chinn and Bob Monroe of
CBS. Howard has in preparation an excellent series
of books on audio engineering which we hope to start
publishing in the near future. From KSL comes word
of a new noise suppressor with many outstanding
features. An article on it will appear in an early issue.
-.I. H. P.
AUDIO ENGINEERING
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FEBRUARY, 1948
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FEBRUARY, 1948
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5
AUDIO SOCIETY ACTIVITIES
Sir
A letter from C. J. LeBel (published in
the January issue of this magazine) explained that a group of audio engineers were
giving serious consideration to the formation of a professional organization for the
advancement of the science and art of audio
engineering. Your readers will be interested to know that this proposed Audio
Engineering Society is organized. Discussions among a group of well -known audio
engineers have already been held, and this
group has appointed C. J. LeBel to be
acting chairman and Norman C. Pickering
to be acting secretary to bring the aims of
this new organization to interested engineers
and technicians. Those who have not been
reached directly are invited to write the
Acting Secretary for further information.
The formal organization meeting will be
held on Tuesday, February 17th, at 8:00
p. m., at the RCA Victor studios at 155 E.
24th St., New York City, at which organizational details will be settled and temporary officers elected. The society has
been fortunate in obtaining Dr. Harry F.
Olson of RCA as the speaker at the first
technical meeting of the Society, to be held
Thursday, March 11th, at 8:00 p. m., at
155 E. 24th St., New York City. Dr.
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FROM PROF. JONES
Dear Mr. Wells:
I have just seen the August number of
Audio Engineering, and have read with
much interest the first of your series of
articles on the design of electronic organs
I hope to see the rest of the articles in
this series, and I congratulate you on the
clear presentation of the material in this
first article.
which the
I notice several points at
statements you have made cause me some
surprise, and in ease you care to know what
they are I am mentioning them in the
following comments.
(1) You state that "the pedal contacts
are usually designed to "make" when the
pedal has been depressed about one inch."
On the manuals you say that "the electrical
contacts "make" when a key is depressed
about one -half inch." These values seem
so large that I have checked them on one
organ. I had no measuring device with
me, but I should say that on this particular
organ the pedals were depressed not more
than half an inch before the pipes spoke,
and the manual keys not more than three
sixteenths of an inch.
(2) You say that combination pistons
are usually actuated "by means of the
knuckles." Last. evening I asked two
different organist, shout t his. Neither
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FEBRUARY, 1948
of them had ever heard of using the knuckles
for this purpose. They said that the
knuckles might, of course, be used, and
that there might be some occasions when
this would be convenient. However, I
doubt it being a practice that is at all
common. If you were playing, let us say,
on the great manual, and wished to change
the combination for that manual, you
would certainly not make use of your
knuckle.
(3) Your treatment of the tremolo on
an organ seems to suggest that it is a
frequency vibrato. In such a stop as a
celeste my impression is that your statement is correct, but is it not the case that
the tremolo on an organ is usually closer to
a fluctuation in amplitude?
(4) You seem to restrict the term
"voicing" to a balance in loudness throughout a given rank of pipes. Is it not true
that this balance is only one of several
things that the voicer must do? Does he
not have the job of making a pipe speak
with sufficient promptness, of seeing that
the speech is good, and of securing a nice
adjustment to the musical quality that is
desired?
I am not an organist, and perhaps my
remarks will be of no value to you, but if
they interest you, you are welcome to them.
A.
When 1948 brings important
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Dear Professor Jones:
I have just returned to my activities
after a long illness; otherwise, I should
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To say that your letter and comments
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(1) Values of Pedal and Manual Movement
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for the distance of travel of they keys.
Organists and designers disagree widely
on the ideal distances.
The values which you give are typical
of certain instruments. Those which
appeared in my article are mean values,
taken from a representative group of
existing instruments. The trend seems
to be toward shorter distances, and it has
been my experience that an instrument
so designed is less fatiguing to play for
long peri/xls.
(2)
events...
Actuating the Combination Pistons by
Means of the Knuckles
I'll admit that this is a technique more
common to theatrical and radio organ ists than to those of the church and
concert fields. Practically all of my
professional work as an organist was in
the entertainment field and, along with
many of my brethren, I used my knuckles
to change combinations.
You are correct in assuming that it
would be difficult to use this method
on the manual upon which you were
playing; that is, with the same hand you
were playing with.
[Continued on page 4.41
AUDIO ENGINEERING
FEBRUARY, 1948
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AmericanRadioHistory.Com
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FEBRUARY, 1948
3
-4
Toscanini conducting the NBC Symphony Orchestra.
Broadcasting Studio Pickup Technique
H. M. GURIN*
Factors influencing placement of microphones and performers in broadcast studios
1ìO('. NI.SS hi the engineering develop-
ment of broadcasting equipment and
studios has been greatly accelerated
in the last decade, and noteworthy contributions have been made in this field.'
As a result, the expectations of higher
standards of technical perfection and
performance may he justified. The usefulness of any improvement is premised
on the skill with which this information
can be applied so that the quality of the
performanceean keep pace with
technical advances placed at one's command.
\ or
In broadcasting, whether it is Al
FM, the primary purpose is to bring to
the listener, in the most pleasing and intelligible manner, whatever information
may be transmitted. For speech, one
would normally look for intelligibility
and naturalness of the reproduced sound
so that a mental picture of the person
and his surroundings may be formed as
well as the message being clearly understood. In music, faithful reproduction
without distortion and the enhancement
of musical programs to heighten the
listeners' personal pleasure are the major
objectives.
The transmission of sound from a
*Engineering Dept., .Valional Broadcasting
Co.
Nygren, A -FM & Television, Mav '46,
Vol. 6, No. 5, p. 25; Volkmann, I.
,Journal of Acoustical Society of America
XIII 234 ('42); Olson, H. F. -RCA Revier',
Vol. 1, No. 4, p. 68 (1937); Olson & Massa,
"Applied Acoustics" P. Blakiston Sons,
PI) il ad) 1phin.
E.-
AUDIO ENGINEERING
broadcasting studio, to achieve the results mentioned above, involves a number
of technical factors among which are:
a. The acoustics of the studio.
h. The electrical system characteristics,
(amplifiers, filters, microphones, etc.).
The studio pickup and microphone
technique.
It is with the last mentioned item that
this article is primarily concerned, and
only some comments will be made of the
first two factors. rinee the program is to
originate in a bro;nlcasting studio of
conventional design, it is assumed that:
e..
(1)
the frequency reverberation
time
characteristic of the space is acceptable
adequate for the
intended programs and audiences, if any,
(3) that the diffusion of the sound field
obtained by proper acoustic treatment
and geometrical configuration is satisfactory and that no unusual grouping of
resonant frequencies exists.2 It is further
assumed that in the electrical system3
(2) that the volume is
' Morse, P. M. & Bott, R. H.-Rec. Mod.
Phys. XVI, 69, 1944,
' NBC Engineering Department Bulletin
-"Down to Earth ins High Fidelity"
O. B. Hanson, C. .4. lackey, G.
TABLE
.. Nixon.-
I
General Properties and Characteristics of Microphones
Model
Type
RCA
44-BX
RCA
77 -D
Ribbon
Velocity
Combination
Ribbon Velocity
and Pressure
Pressure
(moving coil)
Pressure
(moving coil)
Combination
Ribbon Velocity
and Pressure
Condenser
(with assoc.
RCA
88-A
WE
633-A
WE
639-A
WE
Frequency
Response**
30-15,000
+6db
30- 15,000
+5db
Output
Impedance
50/250
+5 db
+3 db
40-10,000
+4 db
50-15,000
±6 db
amplifier)
*Input level of 10 dynes /cm'
* *Manufacturer's specifications
FEBRUARY, 1948
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AmericanRadioHistory.Com
Level*
Directional
Characteristic..
-55 vu
-54.3 vu
-57.3 vu
-60.3 vu
Uni- directional
Non -directional
50 /250
20
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Non- directional
35
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50/250/600
60-10,000
50-9,000
Output
l "ni- directional
Bi-directional
Non -directional
(Amp. Out) Uni -directional
-45 vu
Bi-directional
Non-directional
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250
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1
left
.
Ratio of reflected to direct sound energy with respect
from a ribbon velocity microphone.
to distance
3
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6
DISTANCE
I
Fig.
3
above
.
IO
IN FEET
Frequency response of the RCA Type 44-BX
microphone.
amplitude, two or more microphones
placed at unequal distances from the
source of the waves will receive them successively, rather than simultaneously.
The time interval will cause the coniposite wave to present a different arrangement of its harmonics to each microphone at a given instant. If the outputs
of these microphones are then blended
Stud.o Pickup Technique
of the studio. The directional character and reproduced by a single loudspeaker,
the results manifest themselves as raspiOf the factors listed, probably the most of the sound source and the receptive
controversial is that relating to studio angle of the microphone used as well as ness and raucous tones, particularly at
pickup technique, which includes the ap- its distance from the source are also im- the higher frequencies. It is for this
reason that single mike pickups are recplications and placement of microphones portant.
Fig. I illustrates the relationship of ommended, particularly for musical proand performers. It is important to remember that the system we are dealing reflected to direct sound energy, Er/Ed, grams. However, if multiple microphones
with is monaural and lacks the ability with respect to distance of the sound are required to obtain full coverage, conto discriminate as to the location of a source from a ribbon velocity microphone siderable care must be exercised to avoid
sound, although it can differentiate as to in a suitably treated studio for frequen- distortion caused by wage interference
apparent distance of the source of sound cies between 200 and 1000 cycles. It is and phasing.
readily apparent that any enhancement
to the microphone.
A general understanding of the characWhen sound is generated in a space, of the tonal quality of a singer or instru- teristics of the microphones commonly
of
by
acoustical
properties
mentalist
the
microcollecting
system,
via
the
the
used in broadcasting is of material assistphones, is generally so oriented that the the studio is negligible when the per- ance in selecting the proper type for a
first sounds come from the source directly former is too close to the microphone. specific task to obtain optimum results.
and are followed by the sound reflected Fig. 2 shows the energy response for The tabulation, Table 1, shows the genfrom the surrounding surfaces. When the various microphones in a typical studio. eral properties and characteristics of
Since most musical sounds and human several such microphones.
absorption between boundaries equals
the output of the source, the steady state voices produce sound waves of a corn Figs. 3 -4 graphically show the direccondition is reached. The ratio of re- plicated series of harmonics, each with a tional and frequency response characterflected to direct sound is considered the differ .nt wavelength, frequency and
istics of the RCA -44 -BY ribbon velocity
microphone. Fig. 5 illustrates the RCA Fig. 2. Energy response curves for direct and reflected sounds in a typical studio
77-D combination velocity and pressure
s
microphone characteristics.
The acceptability of the final outcome
MfUCT[D
depends in a large measure on the sul»
NAt
NON -O
MICROPHONE
jective reaction of the individual responsible for the performance as well as on the
of
listener, and for this reason no hard and
fast rules can be established. Instead,
DIRECT
It
some illustrations will be given in which
ANLCii0M6L Ilré117M"iM[
various acoustical problems have been
met and from which general principles
REFLECTED ULTRA 01RECTIORAL
may he derived n- guide to acceptable
I0030
5000
000
300
FREQUENCY- CYCLES PER SECOND
practice
(1) there is no discrimination in any of
the component parts against any frequency within the range under consideration unless specifically desired for special
effects, (2) there is a minimum of phase
distortion, (3) there is a minimum of
harmonic distortion, (4) there is a minimum of extraneous noise.
o--
10
effective reverberation of the collected
sound. It is obvious that an increase in
the total number of reflections reduces
the energy density of each reflection and
permits a more uniform and diffusive
sound field to exist.
The proportion of reflected to direct
sound in a pickup is determined only
partly by the acoustical characteristics
.
761116
AUDIO ENGINEERING
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FEBRUARY, 1948
ratio of reflected to direct sound at the
pickup point. The bi- directional microphone is particularly suitable for a speaker
in a "dead" studio, because the microphone responds to sounds originating
both in front and behind it, thereby increasing the apparent reverberation. Of
course, where the background noise is
excessive or the studio reverberation time
major portion of the sound is direct, as it
should be, when articulation is important,
the reverberation time should be low.
This condition may be carried too far,
and sometimes sounds artificial and unrealistic. Under practical application, the
apparent reverberation may be increased
by increasing the microphone distance
from the speaker, thereby increasing the
When an interview for an individual
is conducted or a brief address is being
delivered from a small speaker's studio,
a bi- directional microphone can be used,
from both sides if necessary. The receptive sides of the microphone should be
located at least 8 feet from the nearest
reflecting wall so that no distortion due
to wave interference results. Since the
CIRCUIT FREQUENCY RESPONSE
OPEN
TYPICAL
MICROPHONE
OF A
TYPE 77 -D
+5
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S
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0
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POSITION
15
60
UNI- DIRECTIONAL POSITION
1,000
100
10000 15,000
FREQUENCY IN CYCLES PER SECOND
OPEN CIRCUIT FREQUENCY RESPONSE
OF A
TYPICAL
TYPE 77 -D MICROPHONE
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0
.
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20
60
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NON - DIRECTIONAL POSITION
Frequency Response Curves
Directional Patterns
Fig. 5.
AUDIO ENGINEERING
Directional patterns and Frequency response curves for the RCA Type 77 -D microphone.
FEBRUARY, 1948
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AmericanRadioHistory.Com
11
sponding characteristic curves. By means
of this unit, a large variety of effects may
he obtained as required by the script.
A very useful adjunct along the same
line is an echo chamber, within which an
additional microphone, either non-directional or bi- directional, and loud -
speaker are located and connected in the
transmission system so that an artificial
delay, together with multiple reflections,
are introduced. The echo chamber is
sometimes used for musical programs,
particularly with small or medium sized
groups where an aural effect of spaciousness similar to a concert hall is to be envisioned. Fig. 7 is a block diagram showing an echo chamber line -up. Fig. S
illustrates a typical arrangement for a
dramatic presentation using an orchestra
for musical bridges, sound effects, voice
effects, together with the usual east and
44-DX
DIRECTIONAL
TYPICAL
VELOCITY
announcer's microphone.
-
CHARACTERISTIC
OF
A
44 -BX
Croups
MICROPHONE
10.000 C.PS.
6.000 C.P.S.
1,000
----
-C.PS.-
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is high, auni -directional microphone which
simulated, as for example, during a telephone conversation or when voices in a
cavern or supernatural voices are being
reproduced in a fantasy or mystery story.
The customary means involve the use of
electrical filters and equalizers inserted
in the microphone circuit. Fig. 6 is a block
diagram of such a device with its corre-
discriminates against all frequencies
equally, other than those in front of it, is
preferable.
Special Effects
Very often, during dramatic sequences,
special speech effects are desired or
Fig. 6 (left). Typical telephone Alter characteristics.
In setting up musical groups, the
technique for microphone placement in
relation to the performers depends on the
type of program, the number of participants, and the effect desired. The pickup
for a solo instrument or vocalist is generally a simple nutter. Care must be
taken, however, not to place the microphone close enough to pick up the mechanical noise of an instrument such as a
piano hammer, the plucked strings of a
guitar, or the surface noise of bowing, as
in the case of a violin. In the case of a
vocalist, it is important to remember
that the low frequency response of the
velocity or ribbon microphone is accentuated when the distance between the
source and the microphone is less than a
wavelength. Consequently, singers
should stay at least 3 feet away or snore,
depending on their volume range. Typical
arrangements for voice with piano accompaniment are shown below in Figs. 7.1
and .9/1.
Fig. 7 (right.) Echo chamber line -up.
1
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AUDIO ENGINEERING
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ECHO CHAMBER
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FEBRUARY, 1948
SOLOIST
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Laudable efforts4 have been made in setting up some mathematical basis for
determining the position but the elements
of personal judgment plus the individual
acoustical character of the space from
which the program originates are too
important to be neglected.
The varying directional characteristics
of the orchestral instruments themselves
must be considered; for example, strings,
woodwinds and percussion are practically
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Arrangement and microphone placement for small musical groups. Fig. 11 (right).
symphony orchestra and soloist.
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Small musical groups, such as quartets
or trios, may be treated similarly to a
soloist with piano accompaniment, with Fig. 8 (right). 'Setup
some slight modifications. In this case, for dramatic (program
0 0
it would be preferable to keep all the with musical bridges.
instruments on the same side of the
0 CAST
Fig. 9 (above).
microphone, as shown in the Fig. 10.
Microphone
placeIf the microphone is located too close ment for solo with
0
to an instrumental group, high differenti- piano accompaniment.
ation of the individual instruments would B-Alternate microPICKUP
result. If the microphone is further away, phone placement fo
solo with piano acANNOUNCE
the sounds will blend together, as they do
MIKE
companiment.
when heard in an auditorium or music
room, and result in a more realistic and
normal reproduction. Care must be
exercised, however, in not going too far
such conditions, the arrangement of the
since "definition" may be lost entirely.
For the concert orchestra, the sensitive instruments needs little, if any, changing
and comparatively noise -free ribbon ve- from the regular concert seating plan for
locity microphone has been highly useful. satisfactory results in broadcasting. IllusBecause of its uniform receptivity at all trated, in Figs. 11 and 12, are the seating
frequencies, it permits greater control of plan and microphone position for the
the low pitched instruments by proper NBC Symphony Orchestra.
angular orientation to the axis of a microThe optimum distance and height of
phone.
the microphone in any pick -up can be
It is possible to bring the entire group determined when all the factors, such as
within an effective angle of 90° by placing acoustical conditions, random noise, size
a single microphone sufficiently far and character of performing group, type
enough in front of the orchestra. Under of microphone selected, etc., are known.
Fig. 10 (left).
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Microphone pickup for small concert
orchestra.
non -directional, while brasses project
strongly in the direction of their bells.
Since the microphone is essentially monaural, it is strongly affected by the directivity of the instruments and since the
apparent volume of sound at a given
angle is inversely proportional to the
distance of the source from the microphone, the strings should be placed nearest and well within the effective response
angle of the microphone in use. On the
other hand, the percussions are not only
non -directional, but capable of almost
unlimited volume. Consequently, they
should be located at the maximum distance and anywhere within the limits of
the response angle. It will be noted that
this arrangement is quite similar to the
usual concert seating plan.
When a soloist is accompanied by an
orchestra, the pick -up for the orchestra
remains the same as described above, but
the soloist may have a separate microphone, and placed so that its position
toward the orchestra is at its minimum
response angle as shown in Fig. 11. Frequently, in the case of instrumentalists
all the instruments cannot be included
within its effective response angle, and
or vocalists with strong, well- projected
voices, additional microphones are not
required, and the orchestra mike serves
as the sole pick -up.
additional microphones are necessary to
obtain full coverage.
Another equally important reason for
the use of multiple microphones with a
dance band is the prominent use of low
volume sounds such as a muted trumpet
or trombone, and other special effects,
which are an inherent part of the musical
content itself. Frequently a rhythm section, consisting of piano, drums, bass viol,
and guitar are grouped together and
separated from the brass and strings.
Because of these special effects, a popular
singer almost always requires his own
microphone. Two illustrations below,
Figs. 14 and 15, show the setup for popular dance bands with and without a
vocalist. In most cases, the special effects
achieved by use of multiple mikes are
considered more important than any
detrimental effects due to wave inter-
Smaller Groups
For smaller groups, such as a salon
orchestra or 20 to 30 piece orchestra, the
fundamental treatment is the same as
previously described. The principles of
directivity and volume of the instruments
must be kept in mind, and the weaker,
non-directional strings, woodwinds, etc..
placed in a correspondingly more favorable location, as illustrated in Fig. 13.
A departure from the single microphone
pickup for a musitpl group is frequently
justified when a popular dance band is
being broadcast. The use of multiple
microphones in many cases is absolutely
necessary. When the program originates
in night -clubs, hotels, ballrooms, etc.,
considerable random noise exists. As a
result, it is necessary to place the mike
as close to the source as possible to exclude the unwanted noise. Because of the
proximity of the microphone to the band,
ference.
The foregoing principles as to methods
and applications constitute only an out[Continued on page 48]
Typical arrangement for dance orchestra. Fig. 15 (right). Microphone pickup for dance orchestra with strings and
Principal microphone. P- Vocalists microphone. Other microphones for group accentuation.
Fig. 14 (left).
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Fig. 13 fright).
Microphone elevation and platforms for orchestra for the NBC Symphony.
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AUDIO ENGINEERING
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FEBRUARY, 1948
Sound Reinforcement in the Hollywood Bowl
M. RETTINGER* and STERLING M. STEVENS **
THE PURPOSE of this article is twofold: (1) To give a resume of the
public address systems employed in
the Hollywood Bowl during the past
nineteen years; (2) To describe in detail
the present installation.
Open-air concerts have long been
popular with the American people. When
the music played in the bandshell of such
an outdoor theater is suitably reproduced
over loudspeakers, it can be enjoyed by
several times the number of persons who
ordinarily could listen to it if it were not
amplified. This fact, plus the concomitant condition that a larger audience
makes, usually, for more popular admission fees, has brought first-class entertainment to untold millions. The electric
ear and the electric mouth are unsurpassed as disseminators of culture.
The "Symphony Under the Stars"
program in the Hollywood Bowl in California is undoubtedly one of the best
known and most popular open-air con*
RCA Victor Division, RCA, Holly -
wood, California.
** Otto K. Olesen 'Company, Hollywood,
California.
The
Describing one of the best
known outdoor orchestra
p-a
installations.
cert series in the world. The number of
famous maestros who have appeared
there outranks that of any other outdoor
concert stage. Celebrated patrons
motion picture stars, music critics, radio
performers, etc. -add a glamour of their
own at these occasions. The Bowl is also
the largest "natural" amphitheater in the
-
world.
The land on which the Bowl is situated
belongs to the county of Los Angeles, and
was leased for 99 years by the Hollywood
Bowl Association 26 years ago. The
Bowl itself is ideally located in the wind protected hollow of some hills dividing
Hollywood and the San Fernando Valley.
It may be remarked that, contrary to
popular opinion, the absence of wind
contributes markedly to the hearing
Hollywood Bowl
quality of outdoor locales. Hearing in the
open is curtailed regardless of the direction of the wind: articulation tests conducted outdoors have shown that a wind
with a velocity of 20 miles an hour generally reduces the percentage articulation
by as much as 40 %, while even gentle
winds having velocities from 5 to 10 miles
an hour, can reduce it as much as 20 %.
Figure 1 shows plan and elevation of
the Bowl, and it is seen that its width is
greater than the length of a football field
and that its total length is nearly 1 /10th
mile. It has a seating capacity close to
20,000-with the spectators in the last
row 120 feet above the boxes in front.
Much of the credit for the development of this cultural center goes to Dr.
Karl Wecker, able manager of the Bowl
and first -class musician.
First Installation
In 1925, seven years after the inaugural
program, the first public address system
was installed in the Bowl. In the light
of modern equipment, the installation
was rather primitive. It consisted of
several dynamic speakers coupled to
at night during an orchestral program.
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auditor was too small to permit ready
localization of the sources. At the boxes,
the auditors had a tendency to look at the
speakers on top of the shell from where
the too powerfully reproduced sound
six -foot long horns, and a power amplifier
containing two UX-250 tubes. A few
years later the reproducers were replaced
by twenty-five 3%' horns disposed about
the lawn in front of the stage. The micro phones-as in many installations of this
originated.
Of course, given time and money, it
might have been possible to develop the
installation into a very satisfactory
system. After two programs, however,
it had to be returned to its owners, and
the Bowl was again without equipment
for the amplification of its music.
But, once more, Leopold Stokowski
came to the Bowl as a guest conductor,
and again he insisted on a reproducer
system. This time-in 1942 -no Paramount Studio and no Western Electric
Company were able to lend equipment.
type at the time -were condenser units,
which, of course, exhibited many of the
defects usually found in the early kind.
Still, the system was able, at the sacrifice of a somewhat restricted frequency
range, to deliver the programs to many
untold thousands who otherwise would
have received it not at all, or else at the
cost of a greatly reduced volume.
When Leopold Stokowski was engaged
to conduct a program in the Bowl, he was
able to promote, through the good will of
the Paramount Studios and the Western
Electric Company, an elaborate "stereophonic" public address installation. Three
complete two-way speaker systems were
placed on top of a 60 -foot steel structure
erected over the music shell, each system
being driven by a separate amplifier. The
positions of the reproducers on top of the
shell corresponded with the sections of the
band below that were picked up by the
microphones in front of the sections. The
amplifier installation, for its day, was
tremendous, consisting of ten 6 -foot racks.
The total effect, however, left much to be
desired. Because the width of the Bowl
is 400 feet, auditors in the extreme side
sections heard chiefly the loudspeaker
system closest to them, thus receiving an
"unbalanced" reproduction of the music.
At a distance of 200 feet in front of the
shelll the "stereophonic" character was
lost since the subtended angle:at the
Fig. 2.
STAGE
,f*
400'
473
-a
a3..
F g. 1.
Layout of the Hollywood Bowl. The
stage for the orchestra is at the lower end of
the drawing.
Block diagram showing arrangement of sound equipment.
PLUG -IN
MUSIC PLUG-IN
MI-11218 A
PRE-AMPS
MI-B218
A
PRE-AMPS
STAGE! MIXER
MUSIC MIXER
0000 0000
0000 0000
0000 0000
BOOSTER
BASS
BOOSTER
EQUAL
BASS
STAGö ER
DRIVER
AMR
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POWER AMP. NO.2
'APRON' SPEAKERS
MI-9119
EQUAL.
MUSICI MASTER
'A'
POWER AMR NO.1
POWER AMP. 140.5
POWER
e
9448
9449
94249
2
9449
9448
With what limited funds were available,
a system was installed which consisted
of a number of high and low frequency
speakers in their respective horns. These
were placed on the ground in front of the
shell, and the sound from them was kept
to a sufficiently low level to achieve a
realistic representation. While, obviously, it was not a superior system, it was
better than none.
In 1945, Leopold Stokowski signed up
as permanent conductor, and purchases
were made of an elaborate RCA installation comprising several two-way speaker
systems, pre -amplifiers, uni-directional
microphones, etc. A forty -foot tower was
built at a distance of 30 feet on each side
of the shell, and a complete two-way
speaker system was installed on top of
each tower. The purpose of this construction was to allow delivery of sound to the
rear sections of the Bowl without flooding the boxes with sound.
This construction also produced a misdirection of sound. This time the extreme
aide-sections were conscious of sound
emanating, not directly from the shell,
but somewhat to the side of it. The central audience area and the boxes were
unaffected. But sound did arrive at the
"gallery!" It was an improvement, a
vast one, and much credit should go to
the maestro who so clearly recognized the
need for amplification. For that matter,
Leopold Stokowski is known to many as
musician who
the sound man's friend
appreciates and understands the technician's problems, and who is sufficiently
acquainted with technical procedures and
terminology to convey to the engineer an
idea of the desired results.
9448
AMP.
NO.4
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SPEAKERS
Final Installation
Finally, in 1947, when more funds became available, a permanent steel structure was built on around the shell so as to
maintain its contour. Near the top of
this structure were mounted two complete RCA two-way systems consisting of
four low-frequency horns and 2 high -frequency horns each equipped with p-m.
units. The horns were so oriented as to
cover with sound the area from approximately 200 feet in front of the shell to the
"gallery." Since localization of sound
was not critical at a distance, these raised
systems fulfilled their purpose very well,
and could indeed he operated at a rather
high level. The sound definitely came
from the shell
certainly gave the impression of coining from there -and the
problem of providing the upper side sections with sufficiently centralized sound
was solved. The area from the shell to a
distance of approximately 200 feet in
front of it was covered with sound by 3
two-way speaker systems installed below
a curved "apron" in front of the shell.
Of course, neither the systems above the
shell nor those below the apron were ex -_
posed to view, but were covered withraa
-it
grill work.
AUDIO ENGINEERING
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FEBRUARY, 1948
e
There is a slight impairment of illusion
for the auditors in the boxes inasmuch
as the frontal speakers are some fifty feet
ahead of the hand, but it is not serious.
It is hoped that next year funds will become available for placement of speakers
below the steps in front of the band shell.
so that even this small shortcoming will
be remedied.
C7
OVERALL
Features
It may now be of interest to explain
briefly some mechanical and acoustical
features of the two-way speaker system
used. The cross -over frequency was
chosen as :300 cycles, rather than suoa
higher frequency, so as to allow a larger
portion of the energy to be issued by the
rather directional high -frequency horns.
This means, of course, that feed -hack
problems are considerably minimized, and
that sound is sent to where it can be
heard and not where it is idly expended.
But what about the feed-back effect
produced by the sound coming from the
non -directional low-frequency horns? This
effect is reduced considerably by a low frequency variable band- suppression filter, having an insertion loss of approximately 10 db over a hand 20 cycles wide.
When this variable band- suppression filter
is adjusted for minimum feed-back, it will
be found that the sound from the horns
can be maintained at a level 10 db higher
O
G
8
1
MICROPHONE
UNI -DIRECTIONAL
MICROPHONE
VELOCITY
MICROPHONE
Fig. 3. Types of microphones used and their placement.
than would be the case without the
equalizer in the circuit.
The system contains also a number of
equalizers which permit the raising and
the lowering of the response above as well
as below 10(0 cycles to provide, if necess:uy, any desired correction, particularly
for voice. Normally the channel characteristic is kept flat, but on occasions
-
Nkaapting.
ANEW SYSTEM for the measurement
of distortion caused by internuldulation has been announced by the
Electrical Research Products Division of
the Western ?lectric Company. Although designecI to determine optimum
processing conditions in variable density
sound -on-film recording, the system is
expected to prove valuable in many- fields
where audio frequencies are used. This
meter replaces the earlier HA -ll07 system which has Iaem in wide use for several
years.
'Ile new system consists of the RA125s signal generator unit and the RA1257 interuaoclulation analyzer. In operation, a signal of two frequencies, the low
between 40 and 1:i0 cycles and the high
either 2000 cycles or between 7000 and
12,000 cycles, are added by the signal
generator with a minimum of :amplitude
modulation of one frequency by the other.
The high frequency is attenuated to a
desired ratio, which may he 1:1, 1:2, 1:4,
or 1:10, and the two frequencies combined in a hybrid coil. Output levels
range between +23 and -44 dhm at 600
ohms output impedance, sufficient for
tests of most equipment without the use
of additional aniiilifiiis which might in
themselves in ta iii iii distortion. The
NON - DIRECTIONAL
always at the direction of the conductor,
the artist, or his manager- compensation
is introduced to achieve time most desirable results.
Figure 2 shows a block diagram of the
amplifier and horn equipment. The two
twelve- position mixers are required clue
to time necessity of having to use the
(Continued on page
.3S
?M4ealiGK
sutinued voltage is passed through the
device under test.
The output from the equipment being
tested is fed to the analyzer where the
percentage of internaodulation, or the
percentage of amplitude modulation of
time high frequency signal by the low
frequency signal, may he closely measured. . distortion phase meter is provided for determining, when measuring
variable density recordings, whether compression is occurring on the positive or
negative half of the low frequency signal.
The analyzer itself functions by first
amplifying the input signal from the device under test, then eliminating the low
frequency component lay means of
band -pass filters. The resulting high
frequency component is amplified and
rectified, producing an envelope which is
a replica of the internodulation in the
input signal. 'fhc average output of the
rectifier is attenuated to a reference
value, amplified, rectified, and applied to
a vacuum tube voltmeter which is adjusted to read the per cent internmdulation directly.
The inclusion of a phase meter in the
Testing with the Western Electric RA -1257
Intermodulation analyzer and RA -1258 Signal
Generator.
www.americanradiohistory.com
AmericanRadioHistory.Com
analyzer, the high sensitivity of that
unit, and the high power output of the
signal generator are among the factors
which will make this new system a valu al le tool in the motion picture field as well
as other fields where audio frequencies
are employed.
Feedback Preamplifier
For
Magnetic Pickups
RICHARD
S.
BURWEN
Complete preamplifier with associated cables.
negative feedback for response equalization, this preamplifier design presents many advantages over previous types.
By using
HE PHONOGRAPH preamplifier described herein shows how a feedback
circuit can surpass conventional design in four ways and actually cost less to
build. These points of superiority are:
1. Very low output impedance enables
use of shielded coupling cable without
causing severe attenuation of high frequencies.
2. Negative feedback reduces harmonic
distortion.
3. Noise and hum originating in the preamplifier are also attenuated.
4. Simpler to build.
Proper reproduction from transcriptions and commercial records with magnetic pickups such as the General Electric
and the Pickering requires that the low
frequencies be boosted with respect to
the middles and, in most cases, that
the high frequencies be attenuated to
offset the attenuation of lows and emphasis of highs put into the record in the
*17 Sheffield Rd., Melrose 17, Mass.
Fig.
1
left
ing characteristics of individual records
can be easily taken care of with the usual
bass and treble controls elsewhere in the
system. By continuing on straight instead of leveling off at 50 c.p.s. it provides
3 db more output at that point than the
N. A. B. playback curve; this helps
compensate for the deficiencies in many
records.
process of recording. Since the output
voltage of these pickups is small, especially at low frequencies, and as it is a good
idea to isolate such low level circuits from
the power amplifier and a-c supply components, a separate preamplifier unit has
been designed which includes this equali-
zation and thereby adapts the pickup to
the medium level input of any flat amplifier. Selective feedback accomplishes
the equalization.
Designed around the G. E. variable
reluctance pickup, the circuit produces
the response characteristic shown as the
smooth curve in Fig. 1. However, it is
readily adaptable to other pickups and
different degrees of high and low frequency compensation with the aid of
formulas developed later on.
The curve in Fig. 1 has been chosen as
suitable for both transcriptions recorded
with the N. A. B. characteristic and commercial records. Variations from the
complement of this curve in the record-
Design Considerations
The conventional method of accomplishing the job of this preamplifier
might consist in using two high -µ triode
stages with a resistance- capacitance equalizer between them. Although this arrangement may be fairly satisfactory
provided all the stray capacities are kept
to an absolute minimum, we immediately
run into difficulty when we try to feed
the output voltage through a shielded
cable to the main equipment, since shunt
capacity in the cable attenuates the high
frequencies.
We could compen-1 to for
required
Play -back curve and its asymtotes for a cross -over frequency of 500 cycles. Fig. 6 (right). Curve showing equalization
to bring magnetic pickup (G. E.) up to flat response on a constant velocity basis.
.
l l
4
s
a01".
k
aooQe
1
1
100
4000
FREQUENCY- CYCLES PER SECOND
18
s
1
10000
1
100P00
1
1
loco
FREQUENCY
-
1
1opoo
CYCLES PER SECOND
AUDIO ENGINEERING FEBRUARY, 1948
www.americanradiohistory.com
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f1
ANCINNEi
this loss within the amplifier,vbut then
we would always have to connect the
amplifier to the sane cable or use a suitable coupling transformer, which would
increase the cost.
Another disadvantage with conventional circuits. when the high frequency roll off part of the equalizer is inserted between stages, is that while tube his,
generated in the first stage is reduced.
that generated in the second stage is not.
A simple way of alleviating both these
disadvantages would be to roll off the
high frequencies at the plate of the second
stage by means of a capacitance to
ground instead of between stages; but
new troubles arise from this expedient
because this reactance, effectively across
the plate load resistor, will seriously
limit the signal voltage handling capability of the tube and greater distortion will
result. If the high frequency roll -off
capacitor were connected between grid
and ground, we have left a new point
where stray capacities and Miller effect
can cause unwanted attenuation of the
highs.
Circuit
All these disadvantages are overcome in
the feedback preamplifier circuit of
Fig. .3. It uses the same number of parts,
but capacitors in the equalizing circuit
are smaller; several more advantages are
incorporated in the design and construction. Feedback from the plate "2 of the
second triode l'2 to its own grid through.
the network consisting of R5, C3, and C4
provides the proper frequency compensation. As a result of this feedback, the
effective output impedance at high frequencies is of the order of a thousand
ohms and the outgoing signal can be fed
through a shielded cable or to certain
types of tone control circuits that ordinarily require a cathode follower driver
without fear of losing the high frequencies. Noise originating in the plate circuit of the last stage is fed back out of
phase to its grid and noise and harmonie
distortion originating elsewhere are reduced by virtue of the frequency discrimination. Harmonic distortion generated
within the second stage is lowered bya factor of 6 at 30 c.p.s. and considerably more
at higher frequencies. Only one point remains where stray capacities can cause appreciable unwanted attenuation of the
high frequencies (the plate P1 of the first
stage) instead of two, and the feedback
eliminates the possibility of high frequency oscillations clue to stray capacitive coupling from plate "2 to the grid
rendering the mechanical layout less
critical.
Ne`lrintssue"AImOe^fa-
Under -chassis view of preamplifier.
inch aluminum can filter capacitor were
mounted on top and the rest of the parts
inside. Filament hum was minimized by
completely eliminating filament wires,
accomplished by feeding the power in
through a miniature four-prong socket
on the side of the chassis that had two of
its lugs soldered directly to the filament
contacts of the tube socket on the top of
the chassis, the types 7F7 and 6SC7 tubes
being conveniently designed with the
filament prongs adjacent to each other.
All the signal -carrying wires were kept
down to less than a half inch in length
and midget coupling capacitors were used
so as to offer as little surface area as
possible to the electrostatic field of the
filament circuit and to prevent loss of
high frequencies through capacity to
ground. Hum is thus reduced to a low
value provided that the center -tap and
not one side of the filament supply be
Fig. 2. Detail of feedback stage. Z represents
the network R5, C3 and C4 of Fig. 3.
R is
approximately equal to R4.
Fig. 3.
Hum
It must be pointed out that no amount
of care in construction can completely
eliminate hum that originates within the
tube itself and that where the utmost of
fidelity is required the six-volt tube should
be replaced by the twelve -volt heater
types to reduce current consumption, so
the filaments may be heated by a 150
ma direct current supply.
Of the tubes indicated the 7F7 is the
one around which this circuit was designed. It was chosen on account of its
high gain and low harmonic distortion,*
the total r -m-s distortion for the preamplifier is estimated to be around 0.1
or 0.2 per cent on peaks. The gain is such
that with the G. E. pickup, instantaneous
peaks seen on an oscilloscope reach five
volts on loud records. However, the
amplification with the type 6SC7 was
found to be nearly the same and the
measured plate voltages turned out to he
a little closer to the desired values. The
6SC7 is also cheaper and, in the cases of
the particular pairs of tubes compared,
less microphonic than the 7F7.
Equalization
Equalization to flat response front the
majority of lateral records and transcriptions requires that the gain rise at a rate
*Sylvania Electric Products Inc., Technical
Manual, Resistance Coupled Amplifier Data
Complete schematic of the magnetic pickup preamplifier, using negative feedback
over the second stage to accomplish equalization.
27K
R7
Ra
1SK
5+ 90 -4501
DESIGN
Ra
1S0K
R2
390
K
GEM
TER 300
V.
Cs
OUTPUT
CI N.aqt
.02P1 500V
0.1
yl
I LOAD R
PREFERNU
PICKUP
INPUT
Construction
The author's preamplifier was built on
a 4 x 2 x 1 inch aluminum chassis with a
bottons cover. The tube and the x :-
1
R1
10
-6SC7
KG
(DUAL
ALL RESISTORS
1
AUDIO ENGINEERING
grounded, and this preferably by means
of a potentiometer.
FEBRUARY, 1948
www.americanradiohistory.com
AmericanRadioHistory.Com
i
OR
MEG.)
7F7'ß`.
TRIODE)
WATT
19
of 6 db per octave below some frequency
which we shall call fi and that the gain
fall off at a rate of 6 db per octave above
some frequency which we shall call f2.
In addition, there may be a frequency at
the low end where the gain begins to
level off again which we will consider
later. Actually, the bends in the curve
at these frequencies are very gradual so
that they can be made with simple resistance-capacitance networks and the slope
only approaches 6 db per octave at a
considerable distance from the bend. The
two frequenciesfr and f2 vary with different manufacturers, and if we were to
construct a network for every combination of fi and 12 in present-day records
we would indeed have a large number of
networks. The simple way out is to build
a single network having f1 and f2 representative of a large number of records
and leave the rest to be taken care of by
ordinary bass and treble balancing controls elsewhere in the equipment. The
smooth curve in Fig. 3 has therefore been
chosen with fr = 500 c.p.s. and f2 = 2000
c.p.s. It corresponds closely to the
present N. A. B. characteristic.
LI
RI
--'WVwHí-
-42
(A)
In (A), network for bass boost only.
In (B), network for
RI at f, (Fig. 1).
R2
frequency attenuation only. Xcz
Fig. 4,
Xc, high
-
at
f_.
-hairiest way to
steps, first by
boosting the lows and then by attenuating
the highs in two separate consecutive
amplifier stages. By replacing our Z in
Fig. 2 with the network A or Fig. 4 we
can attain the 6 db per octave slope at
the low end. The turnover frequency fr
is the point at which the reactance Xcr
= RI. A 3 db rise occurs at this point to
effect the gradual bend, and the slope
approaches nearer and nearer to 6 db
per octave as the frequency goes down
because the impedance becomes very
nearly that of the reactance Xe,, which
doubles in every octave. Above fr the
reactance is small compared to RI and
so the impedance approaches RI, resulting in constant output.
Mathem:atieall,
attain this curve
t
is in
.
t
c
High - Frequency Attenuation
The high frequencies can lie attenuated
at a rate approaching 6 db per octave
above 12 by replacing Z with the network
B of Fig..; where Xc..e = R2 at f2. Below
f2, Xcr lteconies large and the impedance
approaches R2; above12, where the effect
of R2 becomes small, it approaches
Xcr. By passing the signal through a
stage containing network A and then
through a stage containing network B
we get the combination of these two
curves, the same one as in Fig. 1. The
20
straight lines, called asymptotes, show
the limiting value of the slope.
If the phonograph turntable produces
an excessive amount of rumble, common
to the cheap types, it may be necessary
to choose a gain reduction factor of
about 10 for a leveling off point of 50
e.p.s. and in addition lower the values
of all three coupling capacitors Cr, C2,
and C5. The N. A. B. playback curve has
a leveling off point at 50 c.p.s., but this
is usually taken care of by the deficiencies
in the recordings and the associated
equipment, particularly the loudspeaker.
Pickup Response
(I Ici' the actual response of the phonograph pickup has not been taken into
account. The author tested two G. E.
cartridges on the Columbia 10004 -M
frequency record and on H. M. V. constant tone frequency record numbers
D. B. 4034 and D. B. 4035 which according to the label are accurate tu within
0.2 db and whose light patterns tend to
confirm their accuracy. Agreement was
close between the cartridges and fairly
good between the Columbia and the
British records. The general trend of the
curves aras that of a roll -off 3 db down at
3000 c.p.s. reaching a maximum clip of 5
to 6 db and then rising again at 10,000
e.p.s. On the Columbia record 10,000
r.ps. was only :3 db down. Discontinuities in the curve of the British records,
which take four sides to change from
5500 c.p.s. down to 5(X0 c.p.s., prevented
determination of an exact curve for
equalization to flat response, probably on
account of the inability of the large point
-iÍ-a--iEcet
car
RB
vV
(B)
Fig. 5. Network (A) to boost bass and attenuate highs; and (B) equivalent circuit of (A).
radius to reproduce high frequencies as
efficiently at the center of the record as
at the outside grooves.
So far as the author has been able ti
let
determine tine
equalizer curve whirlt comes closest
bringing the output up to flat response is
the one shown in Fig. G. In ordinar
amplifiers it is achieved by means of the
simple 5 db equalizer in Fig. 7 having a
half loss frequency of 25(10 c.p.s. 'l'liicurve seems to be a happy medium si ire
10,000 c.p.s. will he up about 2 db at the
outside of a record and will be down
slightly at the inside.
A word of caution might here be mentioned in connection with the use of long
shielded cables between the pickup and
amplifier. On account of the 100 mh
inductance of the magnetic pickup and
the capacitance of the cable, a ten -foot
shielded cable will add another 2.5 db to
the output at 10,000 c.p.s.
Fortunately, we can compensate for
the pickup characteristic without adding
anything to the preamplifier. It happens
that if we design the equalizing network
for a high -frequency turnover point 12 _
3560 c.p.s. instead of 2000 c.p.s., the curve
of the pickup will subtract from that of
the equalizer and produce almost exactly
the curve we originally intended in Fig. 1.
For the two equalizer networks of Fig. 5
we have.
R..t('..tr = :318 microseconds
RAGA' = 52.0 microseconds
Rj('ar = 274 microseconds
and
R1032 = 44.7 microseconds
The gain
A. =
Zo
R
¡o, now 1336 c.p.s., was chosen to be
slightly less than unity to permit a gain
reduction of at least 50 times and a departure from the asymptote of not more
at
than 0.5 db at :30 c.p.s. for the entire
preamplifier. Network .1 was inure easily
fitted by standard values of resistors and
capacitors, making allowance in CA2 for
the plate to grid capacitance of the tube
and socket.
For use with other magnetic pickups
having a higher output than the G. E.,
it is suggested that the first stage he
eliminated and the pickup connected
directly from ground to the PI side of
coupling capacitor C2 in Fig. 1. The
output can he brought up sufficiently by
selecting a higher value of
-
provided a
higher leveling off point can be tolerated.
Another pickup will of course require
different equalization at the high end. A
worthwhile addition to the circuit that
will make for more pleasing reproduction
from worn records would be a switch
that would shunt a resistor and capacitor
in parallel across the pickup or several
pairs so as to cut off the high frequencies
fairly sharply at selected points in the
manner described in the September 1947
issue of :tndio Engineering. The pre-
i
Fg. 7. Interstage equalizer for flat response
f'om G. E. or similar magnetic pickups. Rg
represents the internal resistance of the driving
amplifier.
amplifier rain lie powered with plate supply voltages other than 3(X) volts; in
fact, there is little difference between
operation at 90 volts and that at 450
volts. But, in general, the higher the
voltage, the less the Ali <tlutimi.
AUDIO ENGINEERING
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FEBRUARY, 1948
Two -Way Speaker System
McPROUD
C. G.
PART
The third of three articles describing the design and
construction of an excellent two -way speaker system.
III
FIRST article of this series
covered the construction of an eight eell multicellular horn for use with an
efficient high- frequency speaker unit,
providing the constructor with information heretofore not commonly available,
and reducing the assembly of a set of
horns to a relatively simple operation.
In the second article, the low-frequency speaker baffle was discussed, together with a number of empirical methods for determining reasonable approximations for the size of a reflexed cabinet,
and the necessary port opening, for
loudspeakers of various sizes. In the
absence of suitable measuring equipment,
it is essential that approximations be
made, primarily because of the difference
in the characteristics of the speakers
themselves, but also to the variation in
preferences of the individual constructor
and the space available for the finished
THE
speaker system.
This article covers the construction of
the auxiliary components required, together with the final assembly of the
system to provide better than average
reproduction from a high-quality sound
source. It will be remembered from the
first article that a complete system consists of a kw-frequency speaker, suitably
enclosed. a high -frequency unit and the
multicellular horn, and the dividing network. In addition, there are a few other
features which make the system more
flexible, and provide sufficient controls
for the critical listener.
Dividing Network
The dividing network consists of two
coils and two capacitors, and serves to
feed the low frequencies to the cone
speaker and the high frequencies to the
horn speaker. There are several reasons
for this -the most important being to
prevent the high -frequency unit from
being damaged by the high amplitude of
the low -frequency excursions which would
be set up in the small diaphragm, inadequately loaded for those frequencies. A
second reason is to prevent the high
frequencies front reaching the cone and
causing the inevitable breakup which
occurs in that type of speaker unless it
is especially designed for the purpose.
The dividing network used with this
"
Managing Editor, Arowo
Eva
AUDIO ENGINEERING
system is the series filter type, with the
circuit shown in Fig. 1. Since both the
specified high -frequency unit and the
low- frequency cone type suggested for
this system were eight -ohm units, the
dividing network is calculated for an
eight -ohm circuit, and the input to the
network should be connected to an eight ohm winding of the output transformer.
The choice of network circuit was
discussed in an earlier article,! and will
not be repeated here. From that article,
however, the formulas for the four cmno-I
N -E
L-2
IB
SPEAKER
ohms)
INPUT
2
e ohms
2
L -P
L
SPEAKER
(8 hhms)
-I
Q9 Q.-
Fig. 1. Basic dividing network circuit used
for this speaker system. This is the series
filter type.
ponents of the dividing net kork are as
follows:
15f1
R
a
ruh
f
L2 = 0.625 L1
CI
nth
159,000
µf
f, Ro
C2 = 1.6 C1
µf
With a crossover frequency of 900 cps and
I"Design and Construction of Practical Dividing .Networks," C. G. McI'roud, Ammo
ENGINEERING,
Fig. 2.
Jane 1947.
an impedance of S ohms, the values are
determined to be as follows: L1 = 1.413
mh; L2 = O.SS3 mh; CI = 22.1 µf;
C2 = 35.3 pf. Taking refuge again in
empirical values, it may he stated that
the two inductances may be made by
winding on forms -4" in diameter and
%" in width, using wood flanges to keep
the windings in place. This size of forth,
when used with No. 17 DCF, wire -as
used in the field coil of Western Electric
55M%í units -will wind about 13 turns
per layer. Referring to the charts of the
earlier article, it is determined that L1
requites 185 turns and L2 requires 146
turns. For optimum results, these should
be adjusted with the aid of an accurate
bridge, but with reasonable care in the
winding the coils to these specifications,
the results should be acceptable.
The accumulation of sufficient capacitance was considerable of a problem until
surplus material became available. The
writer had an arrangement with a capacitor manufacturer long before the war to
provide the values required for dividing
networks at eleven cents per microfarad,
furnished to exact required values. However, since 10-and l5-µf capacitors are
now readily obtainable at six to eight
cents per microfarad, it is much easier
to use these units. Two l0-pf units and
one 2 -µf unit should suffice for the 22.1
µf capacitor, while two 15-µf units and
5-µf unit approach the 35.3 -. f capacitor.
The final values can be built up by the
use of smaller units to the correct values,
using a bridge for the measurement. In
the absence of a bridge, a capacity meter
1
will suffice.
Method of arranging switch sections to support the resistors for the
tenuator
in
FEBRUARY, 1948
www.americanradiohistory.com
AmericanRadioHistory.Com
the
h -f
1
db /step at-
speaker circuit.
21
connected as shown, the total loss is
approximately 12 db -still more than
necessary, but an improvement over the
usual connection.
10 -ke
Supp
When the speaker system is to be used
for reproduction of radio programs from
a high -fidelity receiver, some trouble may
he experienced from the 10-kc inter channel squeal. A simple suppressor can
therefore be installed in the high-frequency horn circuit, and if properly adjusted, it will suppress 10 kc quite effec-
i
rN
b
U
V
ZS0.1S
ou
tl
15
b
Fig. 5. Circuit of 10-kc suppressor useful for
eliminating interstation squeal from wide range radio receivers.
The Complete Circuit
The combined circuit of the dividing
network, the attenuator, the 10-kc suppressor, and the other connections are
shown in Fig. 6. Note that four jacks are
inserted between the output of the switching circuit and the speakers, two in each
circuit. This permits the insertion of a
plug into either circuit for measurement
purposes, or permits feeding a signal to
either speaker without using the dividing
network. This provides sufficient flexibility for the most enthusiastic experimenter
After the completion of the entire
switching circuit, it is advisable to make
frequency- response measurements of both
legs of the dividing network and the 10kc suppressor. The resulting curves
should resemble those of Fig. 9, with the
voltage across the two sections being
equal at points removed from the crossover frequency, and with both outputs
being down 3 db at crossover. The 10-kc
suppressor should be adjusted for frequency by adding or removing turns
from the coil, assuming that the capacitor
values are reasonably close to the specified 0.25 µf in each section. After arriving
at the correct attenuation peak, the
maximum attenuation may be obtained
by an adjustment of the 10-ohm shunting
resistor. When correctly adjusted, the
attenuation at 10 ke should be approximately 40 db, with the response fiat at
about 9,000 and 11,000 and down 6 db
in the vicinity of 9,600 and 10,600 cps.
resistor, 10 -ohms, 10 -watt adjustable,
provides resistance neutralization for the
coil, and makes the attenuation curve of
the equalizer extremely sharp, as shown
by the curve of Fig. 9. This suppressor
should be arranged for switching in or
out of the circuit by means of a switch,
DIallory 2006 -I, push- button type vi jc)
Final Assembly
The dividing network, attenuator, and
10-kc suppressor -if used -should be
mounted suitably on or in the low -frequency baffle cabinet. If the entire
speaker is to be hidden from view, it is
possible to mount these parts on top of
the cabinet at the sides of the high -fre-
tively without appreciably affecting the
remainder of the frequency band.
This suppressor takes the form of a
mull circuit, shown in Fig. 5. The coil is
a 0.5 ntlt unit, composed of 110 turns of
No. 17 DCE wire wound on the same
type of form as used for the dividing network coils. The two capacitors resonate
with the coil, forming a low-impedance
shunt across the h -f speaker circuit. The
Circuit arrangement for h -f attenuator, showing resistor values for an 8 -ohm
circuit.
Fig. 3.
High-Frequency Attenuator
Since the high- frequency speaker unit
is more efficient than the low- frequency
cone, an attenuator will be required in
the h-f output of the network in order to
balance the sound output from the two
speakers at crossover frequency. The
simplest arrangement is to use a 10 -watt
adjustable resistor, with a value of 10
ohms, connecting the speaker unit to the
tap. However, this does not give an
easily variable adjustment, and in general,
those who experiment in audio equipment prefer rather more flexibility than
is afforded by a semi -permanent adjustment.
The recommended high-frequency attenuator is a step potentiometer, with
attenuation values of zero to 6 db in
1-db steps. Such a device is not readily
available on the market, but may be
constructed on a standard switch. The
switch required is a Centralab K-123 index assembly with two "A" decks and
two "B" decks. The switch should be
assembled with the two "A" decks spaced
apart, followed by the two
about 1"B" decks also spaced 1 46" apart, and
with a %" spacing between the two pairs.
This construction, shown in Fig. 2, permits the mounting of 1 -watt resistors
directly on the switch, and parallels two
decks for each circuit, thus increasing the
current carrying capacity. The circuit of
the attenuator is shown in Fig. 3, the
resistors being IRC Type BW -1 in the
values shown. Fig. 3 also shows the connection of the various switch sections.
A simpler arrangement for the high frequency attenuator is to employ a
6-ohm L-pad in the circuit shown in
Fig. 4. The regular I: pads provide attenuation from zero to infinity in their
total rotation, which is more than is
desirable for this application. When
/"
22
locks in either position. Since the suppressor causes a loss of approximately
3.5 db throughout the transmission band,
an additional fixed pad of this value
should be connected across the switch
terminals in the filter -out position, to
equalize the levels. This pad is shown in
Fig. 6, and consists of two resistors. each
being a BW -1, 1 -watt type.
Fig.'4.
Alternate arrangement showing the
of a 6 -ohm L -pad in an 8 -ohm circuit
to provide a maximum, loss of 12 db.
use
ro.
,
0.25
H
0.26
(-0.5
WI
2.7..
0 0
o
o
Fig. 6.
Complete circuit of dividing network, h -f attenuator, 10 -kc suppressor, and jacks to
permit access to the various circuits.
AUDIO ENGINEERING
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FEBRUARY, 1948
quency unit. In any case, the controls
should be accessible, but the special requirements of the physical design may
dictate the actual placement of the parts
and the controls. It should be remembered that the currents involved in a lowimpedance speaker circuit are relatively
large -at one watt, for example, the
current in an ti-ohm circuit is approximately 0.35 amps. On account of this,
the wire used for the connections should
be relatively heavy, particularly when
higher powers are involved. The choice
of switches for such circuits is important
-ordinary toggle switches are not satisfactory for speech circuits at these low
impedances.
The high-frequency horn and unit
should not be permanently mounted until
the exact location is determined. To make
this determination, it is desirable to have
a microphone and an amplifier, together
with an output indicator, for best results.
unit may be reversed, requiring a displacement of 9.2-7.45 in. or 1.75 in.
However, the diaphragm of the low-frequency cone is not a plane surface, and
the exact point of measurement may not
be definite, although there is some evidence to support the choice of the voicecoil position as the measuring point. The
best method of adjusting the two speakers
is by using the microphone, as previously
described.
Actually, a trained ear will suffice to
select the correct location of the two
speakers, although it mayrequire more
time and experimenting. It is necessau;y
to play the same selection from a record
through the speaker over and over. moving the high-frequency horn slightly
each time. The best quality will be
heard when the adjustment is correct.
and after all, the ear is the hest judge of
the performance anyhow.
After the correct position of the:high-
"illl
rallanralliii
MEN
IIIYlß
.1ÍI.
WÌ
Ñ
NIÌ
MEMO
1111111ii
íii
.MIN mm11 nu111
Fig. L. Curves of typical response ho.n dividing network, and attenuation to be expected
From 10 -kc suppressor. Dotted curve shows range of adjustment furnished by 1 db /step
Fig. 7. Furniture cabinet, enclosing same components as used in speaker of Fig. 8. Courtesy
Stephens Manufacturing Co. Inc.
is how well it "wears" although many
authorities advise us that the ear can become accustomed to any speaker, and
when it does, any other reproduction
sounds wrong. However, if the user
snakes a point of attending a live concert
occasionally in order to keen his persjsective on a reasonable balas, s, he should
he able to adjust the operation of a twoway speaker system so that it is capable
of giving increased realism over any
single -unit speaker available. The "presence" afforded by the reproduction of the
frequencies above 900 cps on a small,
well-loaded diaphragm makes listening a
pleasure, and the time and effort spent in
constructing a speaker of this type will be
well repaid.
The use of the attenuator in the high frequency speaker circuit permits a
[Continued on page
attenuator.
With the h -f horn and unit connected to
the circuit, and placed on top of the lowfrequency cabinet, the two should be
energized by a signal at crossover frequency. Then, with the microphone
about six feet in front of the combined
speakers, move the high-frequency horn
and unit back and forth until the maximmm output is obtained, as indicated by
the output meter on the amplifier connected to the microphone. If the position
of the horn is too far forward or too far
back of the front of the cabinet, the two
leads to the high -frequency unit should
be reversed and the procedure repeated.
This will give a new position to the horn
approsiniatel 7.45 in. from the first
position.
The correct position for the h -f unit
can be determined roughly by calculation.
The dividing network used causes a
phase shift of 221° at crossover, which
corresponds to 9.2 in. at 900 cps. Thus
the two diaphragms must he displaced
by that amount. or the phase of the h -f
frequency horn is determined to the
user's satisfaction, the horn should be
permanently mounted. Typical speaker
systems of two-unit construction are
assembled as shown in Figs. 7 and S.
the latter showing a model which may
be used for monitoring purposes, or in
locations where the appearance is not
objectionable. The furniture cabinet of
Fig. 7 includes the sauce apparatus. but
it is all enclosed and suitable for use in the
home. These are conumerchtIiv available
two-way speaker systems.
Operational Readjustments
One of the I,ad features of many twoway speaker systems is that ,the new
experimenter is apt to ruts the system
with the high -frequency speaker operating
at levels considerably above the correct
balance, on the theory- probably--that
"now I have a tweeter and you're going
to hear the high frequencies. or else."
This is a natural fault. and is generally
overcome after using the speaj;er for
some tune. The best test for any speaker
-
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381
Typical mounting of two -way speaker
system, commercially available. Courtesy
Stephens Manufacturing ('o. lac.
Fig. 8.
Facts
About Loudspeakers
O. L.
PART
ANGEVINE, JR.,
I
APERSON interested in sound distribution systems had better be concerned with loudspeakers as he is
going to buy a lot of them. In fact, the
cost of the loudspeakers can be a large
part of the total cost of the sound system.
At the same time, the quality of reproduction of the system can be no better
than its speakers and good results may
be sacrificed to low prices.
Speakers are less well understood than
are electronic devices, such as amplifiers.
because they require a familiarity with
acoustics and mechanics as well as electricity. The use of speakers also involves
the pisychology of hearing and architectural acoustics, which are not part of the
speaker. The fact that the above-men-
'Chief Sound Equipment Engineer, Stromberg ('arlson ('o.
Acoustical Enginrer, Stromberg-Carlson
Co., Rochester, .V. Y.
and R.
S.
ANDERSON
Second in a series for beginners in sound engineering.
tioned intangible factors are involved
allows anyone to think himself an expert on acoustics. It also provides the
acoustical engineer with an alibi in the
face of apparent contradictions, as most
acoustical measurements can be made in
some manner that will permit the desired
interpretation.
Loudspeaker Measurements
Before discussing loudspeakers, it is
important to he familiar with the complexity of their measurements.
The average purchasci of loudspeakers
is not an engineer amp. mturally, is not
familiar with ntagv technicalities of
acoustical measurements as they are
made in the laboratory of the speaker
manufacturer and recorded as "response
curves." Measurements of the frequency
characteristic of an amplifier or an electrical network are comparatively simple
and can he specified in a manner that is
Fig. 1. Outdoor measuring set -up. Two layers of parachute cloth one inch apart serve as a
windscreen transparent to sound. The speaker is mounted on a baffle flush with the wall and
the microphone supported on an adjustable boom in front of the speaker.
universally acceptable. In other words, a
frequency response measurement of an
amplifier, tested in New York, will look
the same as the same amplifier measured
in San Francisco. Unfortunately, there is
no such happy solution for loudspeaker
measurements as there are large variations in observed performance due to the
different methods of measurement and
the differences in the acoustics of the surrounding space. Therefore, it is difficult
for the user to evaluate frequency characteristics from a response curve unless he
is thoroughly familiar with the methods
used to obtain it.
Measurements are preferably made indoors to eliminate dependence on weather
and interference by extraneous noises.
But, in a room, thé sound from the
speaker under test reaches the measuring
microphone not only directly but also
reflected from the walls, the ceiling, the
floor, and from anything else that may be
in the room. The microphone measures
the resultant sounds from many sources,
each different in frequency response, amplitude, and phase -depending upon the
type of reflecting surface and the length
of the reflecting path. Acoustically, a
roost is a hall or mirrors. A listener, because he has binaural* hearing, is able
to discriminate to some degree in favor of
the direct sound over the reflected sound;
but a microphone used for measuring the
response of a speaker does not distinguish
between them. A speaker response curve
represents, as a function of frequency,
the total sound pressure arriving at the
measuring point from all sources and is
not an indication of the impressions of
the listener.
To minimize reflections, the speaker is
measured outdoors (Fig. 1) or in an
anechoic (echo-free or "dead ") room in
which reflections have been reduced to a
minimum by the use of sound -absorbing
material. The response curve of a speaker
depends both upon these acoustic surroundings and the method of measurement- choice of baffle, distance of measuring microphone to speaker, etc.
Binaural-Webster- "Having two ears. " A
person with two ears is able to determine the
direction of a sound source. Binaural is frequent ly used incorrectly to mean stereophonic
or "three dimensional' reproduction. Any
person listening with two ears is obviously
listening binaurally whether or not the reproduction is stereophonic.
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more expensive speaker cannot he justified.
Ì.Ó
SPIFR
12' SPI(R
Fig. 2A.
Fig. 2B.
Directional characteristics of typical 8" and 12" speakers at 2000 cycles.
Directional characteristics of typical 8" and 12" speakers at 6000 cycles.
To obtain the response curves shown
in Fig. 4, a speaker was sent to three
well -known laboratories. Each was asked
to measure the speaker by its normal
methods. As can he seen, the curves are
quite different and might lead to different
interpretations. The differences in response are due entirely to the methods of
measurement, each of which is equally
satisfactory when interpreted by an experienced engineer familiar with the methods and acoustics of that test laboratory,
but dangerous in the hands of people not
familiar with the measuring technique.
To offset this danger to some extent,
manufacturers, when they do publish
response curves, correct for known peculiarities in the measuring set -up.
Point -by-point measurements being
tedious, more response curves per lifetime can be made on a response recorder
such as the one shown in Fig. 3. This
recorder is sponsored by the authors and
does not necessarily represent the opinions of other acoustical engineers.
Other Factors
Frequency response is only one of
several characteristics to be considered
when choosing a speaker for a particular
application. Other factors are directional
characteristics, transient response, intermodulation distortion, harmonic distortion, and efficiency. For many of these
characteristics, there are, again, no
standard invariable methods of measurement.
Directional characteristics are usually
measured outdoors to eliminate reflection
difficulties to which this type of measurement is particularly susceptible. Typical
results are plotted as shown in Fig. 2A.
Note that the 12" speaker is more directional than the 8" speaker. Fig.2/i shows
that both speakers become more directional as the frequency is increased. The reason for both of these facts is that, for any
radiating device to be directional, it must
be large with respect to the wavelength of
the energy it is radiating. Thus, speakers
are non- directional at low frequencies
where the wavelength is many feet and are
quite directional at high frequencies when
the wavelength is only one or two inches.
Likewise, a large speaker becomes directional at a lower frequency than does a
small speaker.
But this measurement sloes not predict
what will actually happen when a speaker
is installed in its normal surroundings.
If a speaker is used indoors, reflections
will var;v the response from point to
point in the room and the total sound
arriving at the listener may be several db
greater than the direct sound from the
speaker. In this case, it is not the onaxis sound pressure that is important. but
the total radiated sound. With speakers
large enough to be quite directional, if
the on -axis frequency response curve is
flat, the off-axis, sound pressure is falling
off at the high frequencies. Thus, the
total radiation is not uniform with frequency as might be thought from seeing
only an on-axis frequency response curve,
but is reduced at the high frequencies.
One solution might he to increase the
on -axis response with frequency. This
might help if one never listened near the
axis, but in the average room, absorption and reflection make both the onaxis response and the total radiation important. A better solution is to use le
directional speakers. As usual, the he-i
answer may be expensive. Another le-expensive solution involves the use of a
well -known illusion. If the response is
peaked in the 1000 to 4000 c.p.s. region,
the listener gets an impression of brilliance similar to that due to a more extended range of high frequencies. This has
become an accepted compromise when the
Fig. 3.
Automatic audio response recorder.
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The absolute efficiency of a speaker is
the total sound power radiated in all directions divided by the total electrical power
available to the voice coil. To secure the
total radiation, it is necessary to integrate
measurements made over the entire
surface of a sphere with the speaker at its
center. This is a laborious process and is
seldom doue. An BALA. committee is
now working on a simpler method to give
an approximate measure of efficiency.
To determine the power handling
capacity of a speaker. it is necessary to
measure the level and frequency at which
mechanical damage occurs. It is also
necessary to determine the level at which
distortion becomes excessive. It is beyond
the scope of this article to discuss the
measurement and interpretational problems involved, but let it suffice that there
is no generally accepted measurement,
Fig. 4.
speaker
curves hom the same
Response
different measuring methods.
(Courtesy Jcnsrrn Radio Mfg. Co.)
Fig. 6.
with
On -axis response of typical 12"
speakers.
and the common practice is to use a
listening test.
Most of the tests described so far are
objective in nature and there is always
difficulty in interpreting objective tests
in terms of subjective results. For this
reason, a listening test of over all speaker
performance should be made in comparison to a known reference speaker. When
making a test of this sort, care must be
taken, to select program material that
covers the frequency range involved.
Broadcast transcriptions may be used
that have been selected for the particular
characteristics of interest. South American orchestras with maracas or popular
dance bands with trap drums allow comparison of the high-frequency response
and spatial distribution. Selections with
string bass and timpani aid in disclosing
cavity resonance and hangover. The test
must be done by skilled listeners and
should not be a snap first -impression test.
FREQUENCY
IN
400 500
700
CYCLES PER
cone speaker radiates the sound directly
from the face of the cone, whereas the
horn speaker couples the diaphragm to
the air through a horn. The cone speaker
is usually used for reproducing speech and
music at relatively low power. The horn
speaker is used to reproduce speech at
high power and to give it directivity.
Construction of Cone Speakers
Modern Speakers
Modern speakers can be divided into
two types -horns and cones (as well as
combinations of them). Horn type
speakers have an efficiency of about 15%
compared to about 5% for cone type
speakers. Both types usually have a
moving-coil dynamic s ,4eme. Intt the
Fig. 5.
300
200
100
Loudspeaker roues arc felted from a
short-fihered material. Details are shown
in Fig. 5. A short-fibered material of the
mixture desired is in suspension in water.
Into this bath, a perforated die, the shape
of the cone desired, is lowered. .1 vacuum
at the Hack of the ,die draws time water
1000
SECOND
through it. depositing the fibers on the
surface of the die. The number and spacing of the perforations and the length of
time the (lie is submerged control the
thickness and weight of the cone. The
next operation dries the cone. At this
point, it is weighed so that not more than
two cones are made before a weight deviation is caught and corrected. A 12"
cone weighs about 12 grants and requires
a tolerance of plus or minus 10 %, so the
process must be controlled accurately.
From this step, the cones go on to be
trimmed to size, lacquer-dipped or treated
in any of the number of ways that may
be required.
.\ spider which holds time Bone and voice
TERMINAL
STRIP
Speaker assembly.
(Below).
DUST
CUP
POLE
PIECE
MAGNET
I
t
RING
FELT
OUST
COVER
26
CONE
SPIDER
VOICE
BASKET
YOKE
COIL
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FEBRUARY, "1948
coil in alignment with the air gap is made
of a plastic impregnated cloth whose
warp and woof are so spaced that air may
pass through but dust and dirt will be
filtered out. A porous spider of this type
r
relieves the back pressure built up by its
motion so that the trapped air can then
escape without being forced around the
voice coil where it may introduce spurious
hisses and noises. Another advantage of
the outside spider over the old center
mounting style, is that it has a high
radial stiffness with great axial mobility.
This means a lower natural resonance and
closer gap clearances. This, with a dust
cap in the center of the cone, makes a dust
and dirt -proof speaker.
The recent introduction of Alnico V as
a magnet material for use in loudspeakers
has served to muddy the waters of an already confusing subject. There is nothing
mysterious about this new magnet material and it contributes nothing to the
quality of the speaker that cannot be
acquired in some other manner. All that
the magnetic structure does is to provide
flux in the air sal) for the wuire nil to
V
operate on. Eight thousand gauss in the
gap is still 8,000 gauss whether it is supplied by Alnico III, Alnico V, or an
electro-magnet field. Of primary importance is the cost of the structure to supply
the required flux density.
Chemically, Alnico V is different from
III in that it contains cobalt and copper
as well as the usual aluminum and nickel.
In addition, it is heat -treated in a manner
that results in a material having about
three times the "energy product" of
Alnico III. The reduced magnet size,
using Alnico V, permits some design improvements. For a direct comparison,
consider the 20 -ounce Alnico III structure.
In current loudspeakers, the
R.11í.
standard 4.64-ounce Alnico V
magnet produces an equivalent flux density. Not all of this difference can be
assigned to the greater energy of Alnico V.
The small size permits a slug- shaped
magnet which appreciably reduces the
leakage flux.
In addition to being more efficient, the
slug-type magnetic structure is less
effec ted by surrounding metal. With a
.
atii4 P4ei4u4e
ling-type magnet, short- circuiting the
magnet has a demagnetizing effect that
may permanently reduce the efficiency of
the speaker. With a slug -type magnet,
shorting bars or mounting boxes only act
as return paths for the flux. This is an
important advantage when speakers are
placed in metal housings.
Rating of Cone Speakers
Hating the fre41ueury response of a
speaker is difficult bcr;nuse there is no
standard. Figure 6 shows the measured
on -axis response of several competitive
speakers. Some of the variations in response in this figure are clue to measuring
conditions as discussed previously. Manufacturers claim different ratings for
speakers of this type. One speaker, no
better than these, is rated to give 14,000
c.p.s. response, although a reasonable
rating would be 50 to 8000 c.p.s. Conservative manufacturers are reluctant to publish ratings until there is a standard,
because the curves on equivalent speakers
will give some justification even to an
[Continued on page
481
24 eteiunifrr.ati4tvs
oecvii>r. 4ici) Qcui
APROCEDURE for measuring the
gain of hearing aids, recently developed by the sound laboratory of
the National Bureau of Standards, offers
to manufacturers and commercial laboratories a useful and economical method
for maintaining adequate quality control
of hearing aids. The apparatus, utilizing
a cavity pressure method,'. permits a
compact test set-up and therefore is much
simpler than that required by the freefield procedure now in general use. Because the equipment may be constructed
at extremely low cost as compared to an
expensive sound -insulated, echoless room,
closer control of the gain performance of
hearing aids should now be readily available even to the small manufacturers.
stimuli available to the user of a hearing
aid. Hence the magnitude of the gain is
directly related to the maximum severity
of hearing loss which 'a hard-of- hearing
person may suffer and still derive benefit
from use of the instrument.
Though the definition of gain is forthright, its measurement is complex and
indirect. It involves measuring the sound
pressure actually impinging on the microphone of the hearing aid, and the sound
pressure produced in the ear of the user
by the hearing-aid receiver.
A major difficulty in testing a hearing
aid and the chief expense for testing
equipment are encountered in the quantitative determination of the sound level
[Continued on page
471
Fig. 1. Laboratory assembly of apparatus used For determining hearing aid gain by a cavity
pressure method. Essential items include (left to right) driver oscillator, the source cavity and
preamplifier source tube, loudspeaker element, electronic amplifier -voltmeter, and in right
foreground, the "artificial ear" and its preamplifier, on which the hearing aid receiver is
mounted. The power level recorder Çright) is used here for automatic recording of data. The
sound pressure level in either the source cavity or "artificial ear" can be recorded by switching the output of one or the other of the preamplifiers into the amplifier and recorder input.
Definition
The gain of a hearing aid, probably the
most important single factor in its performance, may be defined as the ratio of
the sound level transmitted to the ear of
the user by the receiver to the level of the
sound impinging on the microphone of
the hearing aid. The ear of a hard-ofhearing person is less responsive to sound
stimuli than a normal ear, and the gain
is a measure of the magnification of sound
I For
complete technical details see "A
Cavity Pressure Method for Determining
Gain of Hearing Aids," by E. L. R. Corliss
and G. S. Cook, J. Research, .VB.S Vol. 40,
No. 1 (Jan. 1948) RP 1857.
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Elements of Ultrasonics
S. YOUNG
WHITE
A discussion of first principles and methods of making simple ultrasonic generators.
\
1 inquiries have come in from
readers of this series of articles and
from potential users of ultrasonic
energy. There seems to be considerable
confusion in the minds of some as to the
nature of ultrasonics, especially among
those who are not sound engineers. We
have defined ultrasonics in several ways
in this series, but let us re -define it in
terms that will have meaning to, say, a
process engineer who may have only a
dim recollection of his college physics
course.
Ultrasonic energy used in processing
materials is distinguished from ordinary
sound waves as used for communications
by two marked differences: first, the frequency is much higher and secondly, the
energy density is considerably greater.
M\
must be thouscurds of times greater than
this loud sound. In fact, any power density less than about 10 watts/in.2 will
generally produce no effect at all.
Because acceleration increases only as
the square root of the power density, to
increase it by a factor of ten, we must increase our power density a hundred
times. So we must think in terms of tens
of watts or even of thousands of watts per
square inch if we wish to tear material
apart, or seriously change the nature of
some material composed of very fine
particles.
While high acceleration is our chief aim,
there are also doubtless some effects
produced by the large values of sound
pressure we develop in liquids and solids.
For instance, a kilowatt /in.2 in steel
MAGNETO
SMOKE
- STRICTION
OP STEAM
s
PISTON
`GLASS
Fig. 1.
Using
a
magneto -striction ultrasonic generator to coagulate steam or smoke.
This combination produces very high
values of acceleration in the load.
Since all known useful processing effects
stem from acceleration, we aim to secure
the maximum acceleration possible from
a given power. Because acceleration increases directly with the frequency, the
higher the frequency the greater the acceleration. For example, the acceleration is
increased 24 times by simply increasing
the frequency from 1000 to 24,000 cycles.
As for power, an Minch loudspeaker
dissipating one watt of electrical power
in its voice coil delivers only about 50
milliwatts of sound energy spread over
approximately 50 square inches, so the
energy density is about 1 /1000th watt per
sq. in., or one milliwatt /in.2 The ear is a
sensitive device, so if we listen only a few
inches from the cone, the sound will he
very loud indeed.
Power Density
If we wish to produce any noticeable effect upon any material, the power density
*Consulting Engineer, 52 -12 Van Horn St.,
Elmhurst, L. I., N. Y.
28
TUBE
gives us a compression wave of plus
1,000 lb. 'in.2, immediately followed by a
negative pressure or rarefaction wave of
minus 4,(X)0 lb. /in.2, stressing the steel
8,000, in.2 in a rapidly reversing manner.
This effect is independent of frequency.
Some generators we have described have
power densities of over a hundred kw/
in.2, so these values can probably be exceeded in practice. In fact, there is reason to believe we can disintegrate almost
any material in time, as we become able
to develop more power.
Since acceleration equals frequency
times the square root of the power density, to economize on power the must use
the highest possible frequency. But in
practice, there are two modifications
that must be made.
Modifications
The first is that many desirable effects
occur at specific frequencies; for instance.
dust coagulation of one micron particles
requires 24 kc for most efficient results,
and at 100 kc the effect would he very
small.
The second is the energy Mss in the
load. Nature has made nuwy materials
which are almost perfectly elastic -you
compress them and they spring back to
almost original dimensions, less a very
small amount indeed. In radio terms,
their "Q" is very high, and the resistive
component resulting in heat loss is very
low. One standard way to express the
loss in the load is to state how far the
wave will travel until it is weakened or
attenuated to half value.
Water is rather astonishing in that at
24,0(X) cycles the wave will travel about
40 miles before it is weakened to half
power. Most metals have a "Q" of
several thousand. On the other end of the
scale, a 50% solution of cornstarch in
water, stirred for one minute, will reduce
an ultrasonic wave to half power in about
six inches. This brings us to what we
might designate as the cubical nature of
the load, and we can see that it varies
over very wide limits.
The figures on high power density are
rather discouraging when first encountered, as in ninny cases we think of a load
say ten feet square, containing 1(X) square
feet, or 14,400 square inches. At one kw
/in.2, this would mean about 15,000 kw
applied to the load, which sounds quite
impractical. But the "Q" of the load
lowers this figure by a very large amount.
Let us take water as an example:
If we establish 1 kw over one square
inch, we can have a 1 inch square column
of water behind this square inch, and it
can theoretically be 40 miles long if we
are willing to have the far end working
at IA kw per square inch. This gives us
2.4 million cubic inches, or 43 tons of
water hieing treated at one time, and
since many of the effects we desire are
accomplished in a tenth of a second, we
can change this water ten times a second
and treat 430 tons a second. Since in a
practical case we must have boundary
layer losses and wave interferences that
seriously lessen this possible result, we
shall actually treat very much less, but
the figure is so large that even so it is
very encouraging.
Now let us return to the effect of frequency on acceleration. .1t ten times the
frequency, or 240 kc, we would have ten
times the acceleration. Unfortunately,
we would also have ten times the loss
per cycle, and ten times the number of
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AmericanRadioHistory.Com
FEBRUARY, 1948
cycles, so the attenuation would be 100.
The wave falls to half value in 0.4 mile,
and we are only treating 0.43 tons of
material. If we lowered the power to ten
watts/in., we would have the same aceleration, and treat the material ten
times as fast, so if we could prove the
material fast enough past the generator.
we would again have 430 tons a second.
So frequency cancels out if the desired
effect occurs in a thousand cycles and if
the material can be made to flow fast
enough. In general, the advantage of
using the lower frequency and higher
power is that at the present state of the
art it is easier to generate high power at
low frequency than moderate power at
very high frequency.
Since ultrasonic energy is usually useless unless sufficient power is employed
to produce some marked effect on the
material under treatment, and the mini nun power level must usually be determined by trial, let us try to define Uitras(mics from a practical point of view.
Ultrasonic energy consists of alternate
compression and rarefaction waves of
sufficiently high power level to produce
some marked effect in the material,
usually by affecting the particles of which
it is composed.
This definition can be polished up in
time to come, but it indicates there is no
magic in merely increasing the frequency
of a sonic wave -the power must be
enormously increased over that usually
thought of in connection with sound.
'fable I emphasizes the values that
can be obtained in practice with high
power. It is similar to one given
previously, but increased in power and
given in British units throughout.
There is a very good book of its kind,
"Ultrasonics" by Bergmann, published
by .lohn 1Viley and Sons in the U. S. This
book is by all means the most complete
on the subject, and gives 605 references,
the latest being 1938. It is by a physicist
for physicists, and sloes not lean very far
toward the practical. Many papers are
appearing in the Journal of Mc Acoustical
Society, mostly on underwater sound
work clone during the war.
TABLE
luum performance for many uses at
I
Ultrasonic Values
Power Density
Medium
Air
inch, Frequency
100,000 cycles.
kw sq
Pressure Motion
Ib. /inch micro- inches
10
Water
Steel
-1
310
1600
3,000
95
20
4(X0
ke.
-
Acceleration
miles/sac,%
"G"
2,7000,000
8 -1,000
16,800
520
18,000
110
mann whistle can be easily made at home
in a small shop with a lathe. Considerable work can also be done on water jets
interrupted by compressed air bubbles,
as described in the last issue. For commercial equipment we have time following:
Eimer and Amend, (i33 Greenwich St.,
New York City, has two quartz crystal
units--a table model, 200 watts, and a
floor model 500 watts, for about S1,075
and 52.250 respectively. They are pro duced Iq Crystal Research Labs. These
are well made waits, and the danger of
the crystals breaking has been almost
completely removed by good workmanship on the crystal. The oil bath unit
containing the crystal could well be redesigned for special applications. The
range of crystal frequencies is from about
250 kc to over a megacycle, with opti-
The Brush Development Company
has a line of Primary Ammonium Tartrate (PN) crystal units that will stand
temperatures up to 200 degrees F, and
put out from a watt or two up to several
hundred watts, some c.w. and some
pulse rating. The larger units have a
7%2-inch housing, and clue to their small
capacity the voltage is pretty high.
The International Nickel Company
has a bibliography on magnetostriction
which they will supply to anyone interested. So far as the writer is aware, only
Raytheon produces magnetostriction
units commercially, but they are easy
enough for a radio man to construct.
Better have at least 500 watts in the oscillator to drive them. It is a considerable
nuisance to make up the 500 -watt, 20-kc
coils for the job.
A high -power siren good up to about
S kc, as written up in a recent article in
Life magazine, may be inquired about
from the Ultrasonic Corp., 883 Boylston
St., Boston, Mass. My jet turbine unit
is not yet in production for general use.
Anyone who wishes to investigate ultrasonic phenomena in a small way can
[Continued on page 401
Crystal ultrasonic generator providing 600 watts into final stage. Four Frequencies,
kc to 1200 kc, are provided. (Courtesy Einfer (71(11 .1 mend)
Po Nor Level
It must be emphasized that the work
done to elate has been accomplished with
power densities not exceeding 10 watts/
cm2 or 65 watts /in.a. Since many effects
have critical minimum amplitudes that
must he exceeded before any noticeable
effects occur, we can appreciate that the
development of much higher powerdensity generators will allow many
materials to be worked on successfully
that at present show no effects from the
treatment by the 65 watts /in.2.
There has been some interest from
readers asking what ultrasonic apparatus
is available on the American market.
Some simple devices such as the Hart-
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Fro n
400
1948
I. R. E.
National Convention
March 22 -25, 1948
r
THE Institute of Radio Engineers will
hold its 1948 Annual Convention and
Radio Engineering Show at the Hotel
Commodore and Grand Central Palace on
March 22-25.
Theme of the convention and show is
"Radio-Electronic Frontiers," and both the
program and the exhibits are being planned
to fulfill this theme.
A diversified technical program consisting of 130 papers in 26 sessions has been
arranged plus two special symposia with
outstanding invited speakers on "Nucleonics" and "Advances Significant to Elecronics."
The annual banquet of the Institute will
be held the evening of Wednesday, March
24, and the President's Luncheon on Tuesday noon, March 23. Both will feature
national figures as principal speakers. A
cocktail party is scheduled for Monday,
March 22, at 6 P.M.
On the opening morning, March 22, the
Annual Meeting of the Institute will be
held. At this meeting, an innovation at
I.R.E. conventions, Dr. H. B. Richmond
will address the membership on "An Engineer in the Electronics Industry -Prospect, Preparation, Pay."
The largest Radio Engineering Show in
history, occupying two and one -half floors
of Grand Central Palace featuring the
products of approximately 170 exhibitors
will he held in conjunction with the convention. Attendance is expected to equal
or exceed the figure of over 12,000 who attended the 1947 convention and show.
Tentative Technical Program
MONDAY AFTERNOON
MARCH 22, 1948
FREQUENCY MODULATION
"F.\1. Detector Tube kith Instantaneous
Limiting and Single-Circuit Discriminator,"
Robert Adler, Zenith Radio Corporation,
Chicago, Ill.
"A Proposed Combined F.M. and A.M.
Communication System," John C. O'Brien.
"Ratio of Frequency Swing to Phase Shift
in Phase -and Frequency -Modulation Systems," D. K. Gannett and W. R. Young,
Bell Telephone Laboratories, New York,
N. Y.
"A New Magnetron Frequency- Modulation
Method," Philip H. Peters, Jr., General
Electric Company, Schenectady, N. Y.
"I.F. Design for F.M. Receivers," K. E.
Farr, Hazeltine Electronic Corporation,
Little Neck, N. Y.
30
SYSTEMS I
"Technical Aspects of Experimental Public
Telephone Service on Railroad Trains," N.
Monk and S. B. Wright, Bell Telephone
Laboratories, New York, N. Y.
"Reflected -Power Communication," Harry
Stockman, Watson Laboratiries, Cambridge, Massachusetts.
"Static -Free Systems of Detection," D. L.
Hings, International Electronic Corporation, Indianapolis, Indiana.
"Selective-Sideband Transmission and Reception," Donald E. Norgaard, General
Electric Company, Schenectady, N. Y.
"Statistical Methods in the Design and
Development of Electronic Systems," Leonard S. Schwartz, Hazeltine Electronics Corporation, Little Neck, N. Y.
"The Fundamental Principles of Doppler
Radar," Edward Barlow, Sperry Gyroscope
Company, Great Neck, N. Y.
NAVIGATION AIDS
"Tlic ltadiovisor Landing S)stom for Aircraft," Douglas Shearer and William W.
Brockway.
"Considerations in the Design of a Universal Beacon System," I.. B. Hallman, Jr.,
Communication and Navigation Laboratory, Wright Field, Ohio.
"Surveillance-Radar Deficiencies and How
They Can Be Overcome," J. Wesley Leas,
Air Transport Association of America,
Washington, D. C.
"The Course -Line Computer," F. J. Gross,
C. A. A., Department of Commerce, Indianapolis, Indiana.
"Aircraft Instrumentation and Control,"
Francis L. Mostly, John A. Biggs, Earl T.
Heald, and John C. McElroy, Collings
Radio Company, Cedar Rapids, Iowa.
ANTENNAS I
Antennas For Circular Polarization
"Ail Omnidirectional ligli -(ain Antenna
for Circularly Polarized Radiation," A. G.
Kandoian, Federal Telecommunication Laboratories, Inc., Nutley, N. J.
"Analysis of the Effect of Circulating Currents on the Radiation Efficiency in Broadcast Directive Antenna Design," Glenn D.
Gillett, Glenn D. Gillett and Associates
Washington, D. C.
"A U.H.F. Model Study of Current Distributions Induced in Low- Frequency Broadcast Towers and a Discussion of Means for
Reducing Undesired Radiation," Andrew
Alford and Henry Jasik.
"Helical Beam Antennas for Wide -Band,
I
Applications," John D. Kraus, Ohio State
University, Columbus, Ohio.
"Circular Polarization for F.M. Broadcasting," Carl E. Smith, United Broadcasting
Company, Cleveland, Ohio, Robert A
Fouty, O.S.V. Research Foundation, Columbus, Ohio.
TUESDAY MORNING
MARCH 23, 1948
SYSTEMS II
"Thcoretteal Stud) ,I Pulse- Posit iou Modulation Without Fixed Reference," Arnold E.
Ross, St rombcrg Carlson Company, Rochester, N. Y.
"High -Quality Radio Program Links," M.
Silver and H. A. French, Federal Telecommunication Laboratories, Nutley, N. J.
"Signal- to-Noise Ratio Improvement in a
Pulse -Code Modulation System," A. G.
Clavier, P. F. Panter, and W. Dite, Federal
Telecommunication Laboratories, Nutley,
N. J.
"Radio-Wire Links for Multichannel Transmission," E. M. Ostlund and H. R. Hunkins,
Federal Telecommunication Laboratories,
Nutley, N. J.
"Bandwidth Reduction in Communication
Systems," W. G. Tuner, Melpar Incorporated, Alexandria, Virginia.
AMPLIFIERS
"Low -Noise Amplifier," Henry Wallman,
A. B. Macnec, and C. P. Gadsden, Massachusetts Institute of Technology, Cambridge, Massachusetts.
"Phase Distortion in Audio Systems," L. A.
de Rosa, Federal Telecommunication Laboratories, Nutley, N. .1.
"Visual Analysis of Audio-Frequency Transient Phenomena," Donald E. Maxwell,
Columbia Broadcasting System, Inc., New
York, N. Y.
"Square-Wave Analysis of Compensated
Amplifiers," Philip M. Seal, University of
Maine, Orono, Maine.
"A New Figure of Merit for the Transient
Response of Video Amplifiers," R. C.
Palau r and Leonard Mautner, Allen B.
lhunont Lsborat"ri,,. Passaic, N. J.
PASSIVE CIRCUITS
1'1,q,crties of some Wideband Phase Splitting Networks," D. G. C. Luck, Radio
Corporation of America, Princeton, N. J.
"Theory and Design of Constant-Current
Networks," Carl S. Roys and P. T. Chin,
Syracuse University, Syracuse, N. Y.
"New Parameter Adjustment for Television
AUDIO ENGINEERING FEBRUARY, 1948
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Network Transients," M. J. DiToro, Federal Telecommunication Laboratories, Nutley, N. J. R. C. Wittenberg, Ford Instrument Company, Long Island City, N. Y.
"Application of Tehebysehef Polynomials
to the Design of Bandpass Filters," M.
Dishal, Federal Telecommunication Laboratories, Nutley, N. J.
"Matrix Treatment of Four- Terminal Vacuum -Tube Networks," F. D. Bennett,
University of Illinois, Urbana, Ill. J. S.
Brown, Argonne National Laboratories,
Chicago. III.
ELECTRONICS I
Tube Design and Engineering
"Thermiouic Emission from (.:rid., in Vacuuni Tubes," M. Arditi and V. .1. Dc Santis,
Federal Telecommunication Laboratories,
Nutley, N. J.
"The Negative -Ion Blemish in a Cathode Ray Tube and its Elimination," R. M.
Bowie, Sylvania Electric Products, Flushing, N. Y.
"Wide - Tuning - Range Continuous - Wave
High -Power Magnetrons," Paul W. Crapudiet tes, Litton Industries, San Carlos,
California.
"Wide -Range-Tuning Systems for Magnetrons," E. W. Father, Raytheon Manufac-
turing Company, Waltham, Massachusetts.
"Design Characteristics of Hearing-Aid
Tubes," George W. Baker, Chatham Electronics Corporation, Newark, N. J.
ANTENNAS II
"Physical Limitations of Directive Radiating Systems," L. J. Chu, Massachusetts
Institute of Technology, Cambridge, Massachuset t s.
"The Radiation Resistance of an Antenna
in an Infinite Array or Waveguide," H. A.
Wheeler, Consulting Radio Physicist, Great
Neck, N. Y.
"Reflectors for Wide -Angle Scanning at
Microwave Frequencies," R. C. Spencer,
Wade Ellis, and Ellen Fine, Watson Laboratories, Cambridge, Mass.
"Measured Impedance of Vertical Antenna,
over Finite Ground Planes," W. P. Summer,
and A. S. Meier, Ohio State Univier -its,
'Columbus, Ohio.
''Current Distributions on Aircraft Structures," J. V. N. Granger, Harvar l'niver- y, Cambridge,
Massaehustt -.
I
i
4
TUESDAY AFTERNOON
MARCH 23, 1948
SUPERREGENERATION
"So pctic genera
it
Emerges from
World War II," IIarold A. Wheeler, Con ,ulting Radio Physicist, Great Neck, N. Y.
"Theory of the Sup erregeneration Receiver," W. E. Bradley. Phileo Corporation,
t
Philadelphia, Pa.
"Superregeneration-An Analysis of the
Linear Mode," Herbert A. Glucksman.
Watson Laboratories. Cambridge. Mass.
"External and Internal Characteristics of a
Separately Quenched Superregenerative Circuit," Sze -Hou Chang, Watson Laboratories, Cambridge, Mass.
"The Hazeltine Fremodyne Circuit," B. I).
Lmuglin, Hazeltine Electronic Corporation,
Little Neck, N. Y.
TRANSMISSION
"Simplified Procedure for Computing the
Behavior of Multiconductor Lossless Transmission Lines," S. Frankel, Federal Telecommunication Laboratories, Nutley, N. J.
"Optimum Geometry for Ridged Wave guides," W. E. Waller, S. Hopfer, and M.
Sucher, Polytechnic Research and Development Company, Brooklyn, N. Y.
"Fields in Nonmetallic Waveguides," Robert
M. Whitener, Rensselaer Polytechnic Institute, Troy, N Y.
"A Wide-Band Waveguide -Filter Structure," Seymour B. Cohn, Harvard University, Cambridge, Mass.
"Transmission-Line Vector Diagram," W.
C. Ballard, Jr., Cornell University, Ithaca,
N. Y.
NUCLEAR STUDIES
"Oscillator Design for the 130-inch Frequency-Modulated Cyclotron," E. M. Williams and H. E. De Bolt, Carnegie Institute
of Technology, Pittsburgh, Pa.
"An Electronic Interval Selector for the
Determination of the Deadtime and Recovery Characteristics of Geiger Counters,"
L. Costrell, National Bureau of Standards,
Washington, I). C.
"Electronic Classifying. Cataloging, and
Counting Devices," J. Howard Parsons,
Monsanto Chemical Company, Oak Ridge,
Tenn.
"Health Physics Problems in Atomic Energy," K. E. Morgan, Monsanto Chemical
Company, Oak Ridge, Tenn.
"A Selective Detector for Heavy Charged
Particles," Keith Boyer, Massachusetts
r rtst it ll te of Technology, Cambridge, Mass.
ELECTRONICS II
Industrial Applications
and
Electronic Circuits
"I:xperivaental
Study of the
Effects of
Transit Time in Class -C Power Amplifiers,"
Oliver Whitby, Harvard University, Cambridge, Massachusetts.
"New Receiving Tubes for Industrial l-se,"
C. M. Morris and H. .1. Prager, RCA,
Harrison, N. J.
"('se of Diode Rectifiers with Adjustable
Transformers for Motor Speed Control,"
W. N. Tuttle, General Radio Company,
Cambridge, Massachusetts.
"Servo-System Performance Measurement,"
Charles F. White, Naval Research Laboratory, Washington, D. C.
"Spark Oscillators for Electric Welding of
Glass," .lames P. Hooker, Corning Glass
Works, Corning, N. Y.
AUDIO ENGINEERING FEBRUARY, 1948
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Comp
is and Supersonics
"Phase -Corrected Delay bines," M. J.
DiToro, Federal Telecommunication Laboratories, Nutley, N. J.
"On the Theory of the Delay -Line-Coupled
Amplifier," H. G. Rudenberg, Harvard
University, Cambridge, Mass.
"Losses in Air -Cored Inductors," R. F.
Field, General Radio Company, Cambridge,
Mass.
"A Simplified Design Procedure for IronCore Toroids," H. E. Harris, Massachusetts
Institute of Technology, Cambridge, Mas.:.
"Coupling Effects Between Infrared Radiation and a Supersonic Field," W. J. Fry and
F. J. Fry, University of Illinois, Urbana,
Illinois.
TUESDAY EVENING
MARCH 23, 1948
Symposium: Nuclear Studies
A panel uÌ distinguished experts will discuss basic questions in the nuclear field.
WEDNESDAY MORNING
MARCH 24, 1948
Symposium: "Advances Significant
to Electronics"
Five exceptional invited papers from outstanding authors wil'' be presented.
WEDNESDAY AFTERNOON
MARCH 24, 1948
TELEVISION
"A Unitary Tuner-Amplifier for Television
Receivers," E. L. Crosby, Jr. and G. W.
Clevenger, Bendix Radio, Baltimore, Md.
H. Goldberg, National Bureau of Standards,
Washington, D. C.
"A Picture- Modulated R. F. Generator for
Television Receiver Measurements," Allan
Easton, Hazeltine Electronics Corporation,
Little Neck. N. Y.
"The Application of Projective Geometry
to the Theory of Color Mixture," F. J.
Bingley, Philco Corporation, Philadelphia,
Pennsylvania.
"Reflection of Television Signals from Tall
Buildings," Andrew Alford and G. J. Adams.
"Field- Coverage Considerations of New
York Television Stations," Thomas T.
Goldsmith, Jr. and R. P. Wakeman, Allen
R. Dumont Laboratories, Passaic, N. J.
ELECTRONICS Ill
Tube Manufacture
"A>l'M Cotton' 114' Work on Factory Tests
on Cathode Nickel," J. T. Acker, Western
Electric Company, New York, N. Y.
"A Standard Diode for Radio-Tube Cathode
-Core -Material Approval Tests," R. L. McCormack, Raytheon Manufacturing Company, Waltham, Mass.
"European Practices in the Manufacture of
Cathodes," T. H. Briggs, Superior Tube
Company, Norristown, Pa.
"Processing Vacuum -Tube Components,"
[Continued on page 39]
31
eicvldical
Peca2c10s
To Mr. Canby's widely read column, we
add Bertram Stanleigh's interesting evaluation of recent popular music records.
nopuicai 2ec42ts
and angrily return to their old machines.
The gulf widens.
There is much to be done, however,
I've made suggestions right along, as to
available equipment. I am ready to go
farther and give, for what they are worth,
a few ideas for equipment, yet to be
manufactured. Equipment particularly
designed to fit the needs of these earnest,
intelligent non- engineer listeners who can
find nothing to satisfy them for their
money.
Many people insist on the all -in-one
console, in spite of disadvantages, and
this is significant for it must he considered. For them there is the mail order
radio phonograph, at a saving. But
this is no real answer. I have consistently recommended the separate -unit
arrangement. It offers more for the
money, very much greater flexibility,
resistance to obsolescence and performance clearly superior according to engineers' standards, to that of standard
consoles costing the same. For under a
hundred dollars a man can have himself
a good changer, a modest "high fidelity"
amplifier or a P.A. amplifier, and a good
12 inch speaker; fora hit extra there is
the Gl' or Pickering cartridge and preamplifier, or a nylon type crystal. A
piece of wallboard baffling gives as good
results as most confined radio cabinets.
In fact here is surely the ideal basic
equipment to suit in its capacity the real
needs of the Great Gulf consumer.
Except that it has to be put together.
Simple for some, but for the majority
this is an impossible thing! Wires to
hook up, soldering to be clone. A large
number of phonograph owners are ready
and willing to operate more than the
over-simple controls on an average
machine and they appreciate the im-
B\c)
NoW we expect new advances in
a ding to originate in England,
and it will come as no great surprise
that H\IV has announced TT (transient
trite) recordings which include frequencies up to 20,000 cps. The British market
has been strongly affected in the last two
years by the l)ecca ffrr recordings, and
these new TT discs are the answer of the
most important competitor.
That American record companies have
not felt this pressure for wider range recording can be seen from the number of
inferior old recordings which are finding
their way back into the catalog. Gradually they are repressing a large portion
of their prewar catalog of recordings
which are musically interesting but which
could easily he rerecorded under much
finer conditions today. The popular interest in these inferior recordings coupled
with the general lack of interest in the
new Loudon extended range discs seems
to indicate that popular demand will not
he sufficient to force our record companies to improve the quality of their
BERTRAM STANLEIGH**
EDWARD TATNALL CANBY
\III month- ago this department
wade a point of the great gulf that
separates the engineer's conception
of quality audio equipment and the sort
of material commonly used in home
phonographs.
(The observation was
made that, perhaps unintentionally, some
engineers by their very scorn of this
lesser equipment are impeding progress
towards better audio quality in the mass
market phonograph and radio, playing
into the hands of those who capitalize on
the status quo.) I have been a constant
plugger of better quality sound reproduction, in records and in ordinary home
machines too. As a result I am constantly asked by friends and readers to
recommend new equipment, to give them
at least the benefit of some of the improvements I speak of-with their small
budgets.
A few, of course want the best. Recommendations to them are in line with
what engineers might expect, and in their
own terms. But the large majority of
requests -and I am convinced there are
thousands and thousands of other like minded souls-fall squarely in the
middle of the Great Gulf. People who
are sick of their old equipment, dissatisfied with anything in the stores
within their budget (they say so, in so
ninny words) -cyan I recommend any
machine other than those they've tried
can they install one of the new pickups
(hut how does one attach a preamplifier
), would a new speaker do any good?
Radio salesmen are uniformly vague and
entirely uncooperative. On the other
hand, too many engineers, consulted, are
unwilling to compromise with their own
sky-high standards. What to do, then,
given perhaps S75, to a maximum of
S250? Most people, frustrated, give up
Sc
-
...
*17.9 W. 4th
32
St., New York 14, N. Y.
[Continued on page 4.n
nee
products.
The inferiority musically of the first
London recordings to be imported has
created a resistance which will not easily
he overcome despite the vastly improved
quality of their more recent releases. It
seems that only a prolonged record ban
in this country will give these discs
enough of a foothold to favorably influence American recording technique.
One could hardly hope for so drastic a
cure.
The following discs are among the more
interesting in the current crop:
Old Man Rebop London 139. Jack Parnell and his Quartet.
[Continued on page 4.1
*
;1
East 51ah St.,
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FEBRUARY, 1948
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33
NEW PRODUCTS
HOME RECORDER
A new home recording unit with professional features has just been announced by
the Universal Microphone Company, Centinela at Warren Lane, Inglewood, Calif.
Called the Universal RC Recording
Çhassis, this new unit has been designed
and built by commercial recording equipment engineers. According to Mr. Fouch
of Universal Microphone Company, it is
the only home recording unit with patented
and exclusive advantages of:
(1) Recording extremely close tangency.
(2) Patented pantographic movement
makes possible equally spaced cutting
over the entire record.
(3) Groove depth adjustment is visible
and adjustment can be made in recording position.
(4) Pantographic action keeps guide shoe
at correct angle in lead screw thread
-records
inside to outside.
(5) Lift lever at side of head allows the
operator to locate the stylus in the
exact groove location after the lead
screw has been engaged.
(6) RC Recording Chassis records music
ane voice at commercial levels and
loudness.
The maximum frequency range of the
amplifier exceeds 20,000 cycles -with the
Dynamic Noise Suppressor the response is
flat to 10,000 cycles and extends to 16,000
cycles. Independent tone controls allow
boost or attenuation at either end of the
frequency range. A whistle filter is provided
for AM reception. In addition to reproducing phonograph records, the amplifier may
be used with any standard tuner. This
amplifier was designed to provide the best
possible reproduet ion of phonograph records,
FM or AM.
For further data, write Hermon Hosmer
Scott, Inc., 385 Put nam Avenue, Cambridge,
Mass., Dept. AE.
PORTABLE PA AMPLIFIER
A new portable public address amplifier
engineered to provide quality consonant
with the highest price microphones and
loudspeaker systems has been brought out
by Altec Lansing Corporation, according
to an announcement by A. A. Ward, vicepresident. The new amplifier is catalogued
as Model
A -324.
head is automatically
lifted at the end of a 10" record.
(7) Recording
Using a 10" turn table the heavy duty, 110
volt, 60 cycle, 78 RPM motor which can be
easily converted to 50 cycle operations, is
complete with crystal pick -up to play back
12" records.
Further information can be had from
your distributor or by writing direct to the
Universal Microphone Company, Centinela
at Warren Lane, Inglewood, Calif.
SCOTT AMPLIFIER
The mko:ooie -liard -pass principle reaches
new peaks of performance in the Type 210A Laboratory Amplifier. This unit, supplied with a matched variable reluctance
The A -324 is conservatively rated at 15
watts with a guaranteed full power output
within 1 db from 35 to 12,000 cycles. Its
over -all frequency response is rated flat
within 1 db from 20 to 20,000 cycles.
An unusual feature is a continuously
variable bass control which at the low end
is coupled to a switch which cuts in special
equalization to correct for the boomy reproduction which often results from poor
microphone technique. A continuously
variable treble attenuator is also provided.
pickup cartridge, provides a complete
phonograph system except for turntable or
record changer and loudspeaker. The amplifier provides 20 watts output with less than
2% distortion, and below 8 watts, the distortion is under _;¿. The output transformer is arranged to match speaker impedances between 2 and 500 ohms.
34
SHURE WIRE RECORDING HEADS
Them. new recording heads have the
following feature's:
1- Versatility of playback and recording
circuits.
Variety of impedances for individual
needs.
Closely controlled air-gaps for uniform
23-
performance and excellent near characteristics.
4- Reduction of hum pickup.
5- Controlled groove contour for maximum
effective position of recording wire.
For further data, write Shure Brus., Inc.,
225 W. Huron St., Chicago 10, Ill.
HI -FI AMPLIFIER
Allied K:eIio Corporation, Chicago, announces a newly designed Knight 20-watt
phono amplifier that is especially adapted
to high -fidelity reproduction from phonograph records, or AM or FM tuners. Especially suitable for industrial plants broadcasting music programs, for laboratory
testing, music hobbyists, and for all purposes requiring wide range response. Precise engineering has resulted in the production of an amplifier developing less than 2%
harmonic and less than 8'; intermodulation
distortion at rated power of 20 watts. Individual bass and treble tone controls permit
both boost and attenuation of bass and
treble frequencies. With tone controls at
normal, frequency response is plus or minus
Hum is better
1 db from 20 to 20,000 cps.
than minus 75 db from rated output. Gain
is 78 db. Adjustable automatic volume expansion is incorporated. Its action is independent of volume control setting. Dual
high impedance input selected by switch.
Output impedances of 4, 6, 8 and 500 ohm
are provided.
For additional details including complete
specifications and response curves write to
Allied Radio Corporation, 833 W. Jackson
Boulevard, Chicago 7, III.
VOCAL -AIRE SPEAKERS
The formation of a new congtaImv to take
over production of air column loudspeaker,
under the Dilks patents has been announced.
William Petzold and Frank Holdcnecker
are principals of the new orgainzation to be
known as The Dilks Company and located
at Seymour, Conn. Production has been
started and deliveries are being made on
complete sound systems utilizing the Dilks
Vocal-Aire Speaker unit. The new cocn-
AUDIO ENGINEERING
!C,ontinued on page 471
FEBRUARY, 1948
i
« « « « Here at last is a binder using modern
postwar materials at prewar prices. Designed to
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library finish that will stand up under constant use.
Rich blue Dupont Fabricord
stainproof and washable
Rigid backbone with bound -in
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Light
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Coil springs built in at each end
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RADIO MAGAZINES, INC.
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year
MADISON AVE.
AUDIO ENGINEERING
NEW YORK 17, N. Y.
H 1947
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I
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1948
for
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I
AUDIO ENGINEERING
City
FEBRUARY, 1948
www.americanradiohistory.com
AmericanRadioHistory.Com
State
35
HIGH- QUALITY AMPLIFIER
Continuing the discussion of the relative merits of tetrodes vs. triodes, but
with the details of a high -quality tetrode
amplifier to back up his arguments, P.
The amplifier design is relatively conventional, using a pentode for the input
stage, another for the phase inverter,
and two 6L6G's for the output. The feedback loop includes the output transformer,
the connection being made through a
tertiary winding in series with the cathode
8+
OUTPUT
PLATE
of the first pentode. The solution to the
problem of applying a large amount of
t
III
feedback when the output transformer is
included in the feedback loop lies in the
design of the output transformer, which
uses a specially positioned third winding
CORE
for feedback. As designed, the amplifier
performs without any trace of selfoscillation with the maximum obtainable
feedback (using the special transformer)
of 36 db, although the feedback is readily
controllable.
The transformer construction is unique
in the placement of the windings, which
are arranged as shown in the figure. Secw2011a
tions 1 and 7 are the two halves of the
EB.
feedback winding, each half containing
20 turns of No. 27 wire, wound 10 turns
OUTPUT PLATE
on each side of the separator S, and disJ. Baxandall of the Telecommunications tributed over the entire winding area.
Research Establishment presents the data Sections .'..i, 4, and 5 each consist of 800
for another design in the January issue turns of No. 32 enameled wire, with ap-
'il
I
1
r
proximately 70 turns per layer, and with
the layers separated by 1 mil transformer
paper. The 15-ohm output winding,
section 4, consists of 128 turns of No. 20
wire, equally divided between the two
halves of the bobbin. The start leads of
sections 2 and 3 are connected together,
and to the B supply; sections 2 and 5 in
series comprise one half of the plate
winding; sections 3 and B comprise the
other half. The core is a 13/ -in. stack of
Radiometal laminations, 0.015 in. thick.
Coil sections are insulated by three layers
of Empire cloth. Final measurements give
a primary resistance of 160 ohms total,
and an inductance of 60 henrys. The
leakage inductance across the whole
primary with the output winding shorted
is only 50 mh, approximately.
The results obtained with this amplifier give considerable credence to the
belief that the output transformer's
ability to work properly when included
in the feedback loop determines the
operating characteristics of the entire
amplifier.
SYNCHRODYNE RECEIVER
t'ousi,l,rahle interest lets been shown
ill England during the past few months
in a new receiver circuit known as the
Synchrodyne. The circuit was originally
described in principle in Electronic Engineering early last year, and more recently D. G. Tucker and J. F. Ridgeway
have given practical circuits for this
receiver with constructional details in the
:August :mil September issues.
of Wireless World.
The principal reason for the use of
tetrodes is one of economy since the
power efficiency is greater, the required
grid swing is less, and the application of
feedback which is almost a necessity with
triodes also aids in the reduction of hum.
The amplifier described provides 10 watts
into a 15-ohm load from an input signal
of 4 volts rms over a working frequency
range from 30 to 16,000 cps. By working
frequency range, the author implies the
response is down 0.1 db at the indicated
frequencies. The harmonic distortion is
less than 0.1 per cent, at rated output,
and hum is 80 (lb below 10 watts output.
PROFESSIONAL DIRECTORY
J. LEBEL
C.
AUDIO CONSULTANT
Winston Wells
Custom -Built Equipment
370
RIVERSIDE DRIVE
NEW YORK 25, N. Y.
U. S.
1121
PRODUCT DESIGN
PSYCHO ACOUSTICS
ELECTROMEDICAL
36
Recording Co.
Vermont Ave., Washington
5
D
C
INSTRUMENTATION
SOUND RECORDING
SUBMINIATURE AUDIO
District
Designer and Consultant
Acoustical and Electronic Research
307 East 44th St.
MU
4-3487
New York 17, N. Y.
1640
AUDIO ENGINEERING
www.americanradiohistory.com
AmericanRadioHistory.Com
FEBRUARY, 1948
s
The Synchrodyne circuit is not especially new in principle. It consists of an
r -f amplifier and a heterodyne detector.
The new portions of the circuit are the
means used to mix the local signal and
the received signal, and the method of
synchronizing the local oscillator so that
it works at exactly the saine frequency
as the incoming signal.
A simplified circuit of the Synchrodyne
is shown in Fig. 1. One of the principal
advantages of it lies in the divorcement of
the tuning from the r -f amplifier, with a
single tuned circuit being used in the
input to restrict high -level signals from
the grid of the first r -f amplifier tube, thus
reducing the tendency toward cross-modulation. The oscillator is a conventional
three -coil arrangement, with the grid
return being connected to the arm of a
potentiometer across which appears a
portion of the incoming r-f signal. This
provides a means for synchronizing the
oscillator with the signal.
The detector is a ring modulator, and
the output audio signal is fed through a
low -pass filter to remove the unwanted
components. The circuit as shown is said
to give an audio output of 60 my for an
r -f input of 10 microvolts, and the circuit
is capable of operating with increased
r -f input up to an audio output of the
order of 1 volt.
Suitable design should make it possible to gang the r -f tuning capacitor
with the oscillator capacitor, since the
ioniser is not particularly critical. The
volume control as shown is of the feedback type, and it is also necessary to
make adjustments for both r -f sensitivity
and oscillator synchronization voltage.
It would appear that while this circuit
is relatively simple, a few refinements
should simplify the actual tuning operation. If a limiter were to be inserted between the plate of the second r -f tube and
the oscillator, the modulation would be
effectively removed from the synchronizing voltage, with probably better performance. If a limiter similar to those
employed in FM receivers were used, it
would also provide an a- v-c- voltage to
avoid the necessity of manually changing
the gain of the r-f amplifier.
A receiver of this type should provide
a high -fidelity signal if the relative phase
of the two r-f signals can be held at a
reasonable constant value, since phase
shift would cause a variation in a -f output level of 6 db when the relative
phases differed over the range from zero
to 180 °.
A
DYNAMO
AMONG
DYNAMIC MICROPHONES
-
Here is the microphone in its class
a high- output moving -coil dynamic
that was designed to outperform ...
outlast even higher
outsmart
...
priced microphones. The "Sono dyne" features a multi- impedance
switch for low, medium, or high
impedance -plus a high output of 52
db below 1 volt per dyne per sq. cm.
It has a wide range frequency response (up to 10,000 c. p. s.) and
semi - directional pickup. Mounted
on swivel at rear, can be pointed
90° for non -directional pickup.
The "Sonodyne" is ideal for all
general purpose use, including pub-
AUDIO ENGINEERING
Switch for
LOW, MEDIUM,
or HIGH
Impedance
HIGH OUTPUT
(
WIDE RANGE
FREQUENCY
RESPONSE
ing, and similar applications.
Shure Patents Andiny
MODEL
-52 db)
/$04"--*
lic address, communications, record-
(up to 10,000 c. p. s.)
"51"
CODE: RUMON
LIST PRICE . . .
CHANGES IN STANDARD FREQUENCY BROADCAST
Effective .January 30, 1948, the technical broadcast services from radio station WW V of the National Bureau of
Standards will be somewhat modified and
improved, according to an announcement
Multi- Impedance
$37.50
SHURE BROTHERS, INC.
Microphones & Acoustic Devices
225
W. HURON
ST.,
CHICAGO
FEBRUARY, 1948
www.americanradiohistory.com
AmericanRadioHistory.Com
10,
ILL.
CABLE
ADDRESS:
SHUREMICRO
37
by Dr. E. U. Condon, Director of the
Bureau.
Each of the eight radio carrier frequencies 2.5, 5, 10, 15, 20, 25, 30, and 35
megacycles will be broadcast continuously
day and night. Standard audio frequencies of 440 and 4000 cycles per second will
be transmitted on the carriers 10, 15, 20,
and 25. The 440 cycle frequency, which is
the standard of musical pitch (A above
middle C), will also be broadcast on 2.5
and 5 megacycles. The accuracy of each
of the transmitted radio and audio frequencies is better than one part in 50
million.
The attention of all users of the
National Bureau of Standards time an-
nouncements is particularly called to the
following change: Time announcements
in International Morse Code, accurately
synchronized with basic U. S. Naval
Observatory time, will be advanced one
minute with respect to the old announcement scheme. With the new system the
audio frequencies are interrupted at
precisely one minute before each hour
and at each succeeding five -minute
period. They are resumed precisely on
the hour and each five minutes thereafter.
Under the old system, the time signals
were interrupted for a minute on the hour
and on each succeeding five minutes,
while under the new scheme interruptions
will he for a minute precisely on the 50th
ealo
'µ
Ingenious New
Technical Methods
To Help You
Simplify Shop Work
Two -Way Speaker
Metal Turning Made Easy
with New Simplified Tool!
now tool called "Tru- Turn" makes possible the conversion of drill presses, woodturning lathes, or grinder
stands into tools that will turn and cut -off steel, bronze,
copper and aluminum. The "Tru- Turn" tool shown above
is mounted on a Buffalo Drill Press, Spindle Size.
The "Tru- Turn" tool is easy to operate and cuts and turns
bar stock of steel, bronze, copper and aluminum measuring W., 3/8' and W. Its built -in micrometer permits adjustments that give tool -room accuracy to 1 /1000 inch.
Small tool shops as well as all types of repair shops and
garages find the "Tru- Turn" ideal for cutting long pieces
of bar stock into desired lengths. Also, home craftsmen
are able to produce accurate, highly finished precisionedmachined parts from metal even without previous training.
Accurate, precision work is also easier to do when tension
is relieved by chewing gum. The act of chewing gum
seems to make the work go easier, faster -thus helping
on- the -job efficiency. For these reasons Wrigley's Spearmint Chewing Gum is being made available more and
more by plant owners everywhere.
[from page
Tru -Turn Tool
You can get comp /ere information from
Screw Products Corp., 132 West 13th Street
,lliilbolland
2,
Ind.
AC-55
38
231
small variation in the level of the frequencies above crossover, and this will be
set at a point that appears to give the
correct balance. The average difference
between the two outputs is of the order
of 4 to 8 db, and with the 3.5 db fixed
pad to replace the 10-kc suppressor when
it is out of the circuit, this indicates that
from 0.5 to 4.5 db will be required in the
variable pad. It may be desirable to renieve the suppressor from the circuit
when using the speaker for reproduction
of phonograph records, but in general, the
band suppressed is so narrow that it is
difficult to detect, and little harm is done
by leaving it in the circuit at all times.
It does help on AM radio, however, when
the output of the tuner contains any of
the objectionable squeal which goes with
the usual high-quality tuner.
A
Indianapolis
minute, on 4 minutes past the hour, 9
minutes past the hour, etc., and resumed
precisely on the hour and each five minutes thereafter. The exact moment to
which the time refers is the moment of
interruption of the audio frequencies of
440 and 4000 cycles per second. The audio
frequencies will continue to be interrupted
for one minute to allow for the time announcement, for station identification
by voice at the hour and half hour, and
to afford an interval for checking radio
frequency measurements free from the
presence of audio transmissions.
Station WWC provides six important
technical broadcast services to the nation
and five to the world, 24 hours a day.
These are: (1) standard radio frequencies,
(2) time announcements, (3) standard
time intervals, (4) standard audio frequencies, (5) standard musical pitch. (6)
radio propagation disturbance warning
notices. The national standard of frequency, of which the Bureau is the custodian, is fundamental to much of the work
in radio, electronics, acoustics, and other
fields where measurements require accurate frequencies. Accurate time-interval
signals are important in seismology. geodesy, navigation, and research. The
Bureau's broadcasts -the only suds service being provided by any country -are
being utilized by many organizations including schools and universities, the Department of National Defense, scientific
laboratories, manufacturers, amateur
radio operators, observatories, power
companies, communication companies,
musicians, and radio broadcast companies.
A detailed announcement of WWV
broadcast services, LC886 will be provided upon request from the National
Bureau of Standards, Washington 25,
D. ('.
The first and attend instalments of this aeries
appeared in the November and December,
/947, issues of .AUDIO ENGINEERING.
AUDIO ENGINEERING
www.americanradiohistory.com
AmericanRadioHistory.Com
FEBRUARY, 1948
f
1948 I.R.E. Convention
Valzia,41e
Paul D. \1 illi;un, Eitel- McCullough, Inc.,
San Bruno, Calif.
"Continuous Exhaust Machine for Electronic Tube Manufacture," L. Grant Hector, Monotone Corporation, Elmsford, N. Y.
EQURLIIER
LINE
EQUALIZATION FOR VARIOUS
CONTROL SETTINGS
600 -OHM SOURCE AND LOAD
o.
I=i111111A1
MEASUREMENTS I
V.H.F., U.M.F., and S.H.F.
"Swept- Frequency 3-Cm. Impedance Indicator," H. J. Riblet, Submarine Signal
.
FEATURES
iiiií
Elilal
Adjustable to equalize at
either 8 or 10 kc
somosoasMI011M
Ls-
Company-, Boston, Mass.
JIIIII- i!011111
"An Automatic V.H.F. Standing -WaveRatio Plotting Device," W. A. Fails, L. L.
Mason, and K. S. Packard, Airborne Instrument Laboratory, Mineola, N. Y.
"Microwave Impedance Bridge," M. Chodorow, E. L. Ginzton, and J. F. Kane, Stanford University, California.
"Impedance Measurements by Means of
Directional Couplers and a Supplementary
Voltage Probe," B. Parzen, Federal Telecommunication Laboratories, Nutley, N. J.
"A Waveguide Bridge for Measuring Gain
at 4000 Megacycles," A. L. Samuel and D.
P. (' randoll, University of Illinois, Urbana,
Continuously variable
equalization control
Engraved scale for easy
resetting
Three -hole mounting
111
1101
COO
The new Type 402 Line Equalizer combines the lower
cost of fixed units with the flexibility of variable
Five
control. Calibrated dial permits quick adjustment of
resistance value when panel mounted for use on
various lines, or as indication of setting when permanently connected to individual line. Resonant
frequency may be set at either 8 or 10 kc by strap
on terminal lugs to provide optimum adjustment for
flat equalization over entire audio band.
Low in price
THURSDAY MORNING
MARCH 25, 1948
Type
402
i .íi: ;..+¡i....
._.
:Fa
.
units mount on a
single 31/2" panel
Works on lines from 150
to 600 ohms
: ti' '
III.
COMPUTERS
-
'
;.flt:rr,...S.,7v:,--_
I
Systems
-I.:tr,. .Shalt Computers," R. L. Snyder,
University of Pennsylvania, Philadelphia,
Pennsylvania.
"The Univac," J. L. Mauchly.
"Engineering Design of a Large Scale Digital Computer," James R. Weiner, Charles
West, and John E. De Turk, Raytheon
Manufacturing Company, Waltham, Mass.
"A Network Analyzer for Study of EIeemnagnetic Fields," Karl Spangenberg,
t hnn Walters, and F. W. Schott, Stanford
oi,rsity, California.
c
--_ -_-
$15.00
.;-
Radio Engineers! Hams!
If you are
interested in amateur
radio, you'll want
"CO"
month.
Subscribe now.
handy
coupon
every
Use the
I
Broadcasting and Recording
"Modern Design Features of CBS Studio
Audio Facilities," R. B. Monroe and C. A.
Palinquist, Columbia Broadcasting System,
Inc., New York, N. Y.
"Methods of Calibrating Frequency Records," H. E. Roys, R. C. Moyer, and D. R.
Andrews, RCA, Camden, N. J.
"Distortions in Magnetic Tape Recording
Due to the Configuration of the Bias Field,"
S. J. Begun, The Brush Development ComPany, Cleveland, Ohio.
"Instantaneous Audience Measurement System," Peter Goldmark, Columbia Broadcasting System, New York, N. Y. John W.
Christensen, Andrew Bark, John T. Wilmer.
Propagation
"Continuous Tropospheric Sounding by
Radar," Albert W. Friend, RCA, Princeton, N. J.
".4 Theory of Radar Reflections from the
AUDIO ENGINEERING
below.
Attach
your remittance and mail today.
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FEBRUARY, 1948
www.americanradiohistory.com
AmericanRadioHistory.Com
Zone
_..__
State
....__
39
A BASIC Improvement
in Sound Reproduction
KLIPSCH
SPEAKER SYSTEM
FREQUENCY RANGE
30
ELECTRONICS IV
to 15,000 cycles
PERFORMANCE equivalent to
conventionally d e s i g n e d
speakers 8 to 16 times as
bulky
Selected by Major
E. H. Armstrong for his two
most important demonstrations of FM before the I.T.C.
and N.A. 8
conventions.
FOR
INDISPENSABLE
Monitoring
Broadcast Station
High Fidelity Radio-Phonographs
Electronic Musical Instruments
(Organs etcl
High Quality Sound Reinforcement
Systems
Wired Music Installations
Research, Test Work, Demonstrations
Wide Range Reproduction
of
The KLIPSCH Speaker System design utilises the
corner of a room as an integral port of the acoustic
system, the walls and floor being in effect an extension of the low frequency horn
FUNDAMENTAL TONES
down to
30 cycles per second.
CLEAN RESPONSE
throughout
the
range of hearing
LOW DISTORTION
and intermodu-
lotion at oll frequencies.
PERFECT DISPERSION
of middle
and high frequencies throughout the entire room.
HIGH
EFFICIENCY: Because of the
horn loading, acoustic output for o given input
power is several times that of conventional
speakers.
NON- RESONANT: BASS TONES
ARE REPRODUCED - not generated
by the speaker. Instruments of low pitch are
clearly recognized; one hears the original tone
not one created by the loudspeaker.
-
ASK FOR FULL PARTICULARS
Dept. A
BROCINER
1516 SECOND AVENUE, NEW YORK 28,
40
LABORATORY
N
Y
Lower Atmosphere," W. E. Gordon, University of Texas, Austin, Texas.
"New Techniques in Quantitative Radar
Analysis in Rainstorms," David Atlas, Air
Material Command, Wilmington, Ohio.
"The Propagation of Radio Waves Through
the Ground," Knox McIlwain, Hazeltine
Electronics Corporation, Little Neck, N. Y.
Harold A. Wheeler, Consulting Radio
Physicist, Great Neck, N. Y.
"Design and Application of a Multipath
Transmission Simulator," Harold F. Meyer
and Arthur H. Ross, Coles Signal Laboratory, Red Bank, N. J.
REgent
16194
New Forms of Tubes
"Nest Design for a Secondary- Emission
Trigger Tube-NUTR- 1032 -J," C. F. Miller
and W. McLean, National Union Radio
?1''t
Corporation, Orange, N. J.
"A Spiral -Beam Method for the Amplitude
Modulation of Magnetrons," J. S. Donal,
Jr. and R. R. Bush, RCA, Princeton, N. J.
"TheeDyot.ron -A New Microwave Oscillator," E. D. McArthur, General Electric
Company, Schenectady, N. Y.
"Electrostatically Focused Radial -Beam
Tube," A. M. Skellett, National Union
Radio Corporation, Orange, N. J.
"A New Two-Terminal High-Voltage Rectifier Tube," George W. Baker, Chatham
Electronics Corporation, Newark, N. J.
MEASUREMENTS II
"SimpliÍir:,tion of the Theory of Supersonic
Interferomel ry," J. L. Hunter, John Carroll
University, Cleveland, Ohio.
"Frequency Measurement by Sliding Harmonics," J. K. Clapp, General Radio Coinpany, Cambridge, Massachusetts.
"A General- Purpose Oscillograph for Precision Time Measurements," R. P. Abbenhouss, Allen B. Dumont Laboratories,
Clifton, N. J.
"Some Considerations in Extending the
Frequency Range of Radio Noise Meters,"
W. J. Bartik and C. J. Fowler, University of
Pennsylvania, Philadelphia, Pa.
"Some Considerations in the Design of
Precision Telemetering Equipments," R.
Whittle, Federal Telecommunication LabJ
oratories, Nutley, N. .1.
THURSDAY AFTERNOON
MARCH 25, 1948
COMPUTERS II
Components
"Megacycle Stepping Counter," Charles B.
Leslie, Naval Ordnance Laboratory, Washington, D. C.
"Rectifier Networks for Multichannel
Switching," N. Rochester, Sylvania Electric
Products, Inc., Boston, Massachusetts, D.
R. Brown, Massachusetts Institute of TechI,tlogy, Cambridge, Massachusetts.
\lerrury Delay -Line Memory Using a
Pulse Rate of Several Megacycles," Isaac
L. Auerback, J. Prosper Eckert Jr., Robert
F. Shaw, and C. Bradford Sheppard, Electronic Control Company, Philadelphia, Pa.
"Selective Alteration of Digital Data in a
Magnetic Drum Computer Memory," A. A.
Cohen, W. R. Keye, Engineering Research
Associates, Inc., St. Paul, Minn.
"Methods for Visual Observation of Patterns Recorded on Magnetic Media," S. N.
Alexander, L. Morton, I. L. Cooter, National Bureau of Standards, Washington, D. C.
Microwaves
"Cavity Resonators for Half -Megavolt
Operation," A. E. Harrison, Princeton
University, Princeton, N. J.
"Analysis and Performance of Waveguide
Hybrid Rings for Microwaves," H. T.
Budenbom, Bell Telephone Laboratories,
Whippany, N. J.
"Frequency Stabilization with Microwave
Spectral Lines," W. D. Hershberger and L.
E. Norton, RCA, Princeton, N. J.
"Analysis of a Microwave Absolute Attenuation Standard," Anthony B. Giordano,
Polytechnic Institute of Brooklyn.
"Synthesis of Dissipative Microwave Networks for Broad-Band Matching," Herbert
J. Carlin, Polytechnic Institute of Brooklyn.
"10 -Cm. Power-Measuring Equipment,"
Theodore Miller, Westinghouse Electric
Corporation, East Pittsburgh, Pa.
Receivers
"The Application of Noise Theory to the
Design of Receivers," William A. Harris,
RCA, Harrison, N. J.
"The Design of Input Circuits for Low
Noise Figure," Matthew T. Lebenbaum,
Airborne Instruments Laboratory, Mineola,
N. Y.
"Frequency Converters," William H. Lewis,
Pennsylvania State College, State College,
Pa.
"Radio Set AN /CRD -1," William Todd,
Evans Signal Laboratory, Belmar, N. J.
Active Circuits
"Reactance-Tube Circuit Analysis," R.
Carroll Maninger, C. S. Navy Electronics
Laboratory, San Diego, Calif.
"Electronically Controlled Reactance," J.
N. Van Scoyoc and J. L. Murphy, Armour
Research Foundation, Chicago, Ill.
"Stable Regulated Power Supplies," Robert
R. Buss, Northwestern University, Evanston, Ill.
"The Photoformer," D. E. Sunstein, Philco
Corporation, Philadelphia, Pa.
"Mode Separation in Oscillators with Two
Coaxial -line Resonators," Herbert J. Reich,
Yale University, New Haven, Conn.
Ultrasonics
frein page "r
do so with quite simple equipment. A
pair of rubber microphones, a Hartmann
whistle, a Rochelle salt crystal from an
old phonograph pickup and a simple low
powered nickel magnetostriction unit will
allow considerable work to be done.
The Navy surplus R. \K -5 receiver, or
Letter yet the DZ -2, with a tuning range
I
J
AUDIO ENGINEERING FEBRUARY, 1948
www.americanradiohistory.com
AmericanRadioHistory.Com
f
of
14 to 1600 kc makes an excellent vacuum tube voltmeter, sensitive down to a
microvolt, and being sharply tuned, can
be used as a harmonic analyzer as well.
The signal- to-noise ratio of these two
receivers is very good.
Do not try to do much with the quartz
crystals available from surplus. It is
true that some of the larger transmitting
crystals have good activity, but they are
usually shear cut, and only X cut types
give a disciplined piston action. The
shear cuts have activity on the edges in
a rather peculiar manner that makes
their emitted pattern very complex
and almost unusable.
The phonograph pickup types are
usable even if broken if you clear away
any tinfoil causing a short. The full
size job will act as a microphone with
many resonant spots up to well above
100 kc, and small pieces, say one -eighth
inch square, may work above 250 kc. If
used under water they may be rubber
protected, or given a glyptal coat. If
used in air, be sure they are lined up
toward each other or the source, as their
pattern is very sharp.
A word of warning if you go in for
magnetostrictivn-lie sure you have "A"
or "Z" nickel and not monel, which is
almost useless. If you have access to a
little electric furnace, treat the rod or
tube for several hours at 1475 degrees F.,
and then turn off the furnace and allow
at least 12 hours for slow cooling. Be
very careful not to shock the rod in
handling. If you even chuck it in a lathe
very carefully and lightly polish it with
crocus cloth, you will lose most of the
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Smoke Coagulation
A magnetostrictive cooled tube Makes
a fairly satisfactory device for setting up
resonant waves in air to observe coagulation of smoke. The trouble with using a
whistle for this work is that the percentage of modulation of the air stream
is only about 4%, so a large quantity of
useless air appears as a blast that blows
away the smoke you wish to study. We
can use a parabolic reflector and bring
the energy to a focus say 7 feet away, so
most of the air blast will disappear and
there will be high energy density at the
focal point.
If you take the magnetostrictive rod,
or better yet, use a tube and water-cool
it, then you can sweat a 1%2 --inch plate on
one end, say three-sixteenths inch thick.
Take a precision glass tube that clears
this diaphragm by a few thousandths all
around, and mount it so the diaphragm
is well into one end. Arrange a movable
piston at the other end and adjust it to
resonate the air inside the tube, thus we
can build up quite high field strengths
that will coagulate tobacco smoke and
steam inside the tube. See Fig. 1.
V'autinaed an page ¡_'!
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FEBRUARY, 1948
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AmericanRadioHistory.Com
41
To digress for a moment, several friends
of mine have made up little Hartmann
whistles to call their dogs by ultrasonics.
They all had some difficulty in adjusting
the spacing to give oscillations, so they
claimed. The real reason is that the air
in the jet must reach sonic velocity in
HARVEY
has
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42
Popular Recordings
front payr
s
I
One of the finest discs yet imported by
London. The Parnell Quartet has produced some palatable rebop which might
even teach some of its American creators
3 tiling or two. Technically this disc is
order to "pack" the air into the resonant
cavity. This takes at least 13.6 lbs./in.2
;ol absolute dream. It is unmonitored
of air, and nobody except Superman can
;unl Ilan a very wide frequency range.
Dickory
Dock 'London 137. Ted Heath
develop over 4 pounds pressure, so it is
Isis ylusic.
impossible to blow a Hartmann whistle and The
Ted Heath band is the larger unit
by lungpower.
from which the Parnell Quartet stems.
In order to make an intelligent effort
Like the quartet, they seem to have
grasped the rhythmic quality of American
to analyze a load from the commercial
popular music. The results are good, and
point of view, there are four things we
this disc compares favorably with the
must know. \Ve shall call them Z, G, Q
best dance music currently produced here.
and T. These represent Impedance.
Reproduction is tip to the ffrr standard.
Old Time Religion Victor 20-2614. Phil
Acceleration, Losses, and Time of ApHarris
rris anti Ow Sportsmen.
plication of the (lower.
The musical material on this disc is a
Z is the sonic impedance. If the load
cheap, condescending imitation of a negro
spiritual, but the recording is one of
is a gas, we must use a gas -type generaVictor's most successful in the popular
tor. If liquid or solid, we can probably
field. It is smoothly monitored, and aluse either a liquid or solid generator and
though the range does not extend beyond
match the impedance to suit.
6,000 cps., there is excellent. clarity and a
acoustical effect.
pleasing
"G" is the acceleration required to produce some desired effect. In coagulation
of aerosols or dispersoids we really work
Hollywood Bowl
by the length of path of the excursion of
from page 171
the particles, so that many collisions take
place in a short time. Most other effects
multiple- microphone method of pick -up.
stein from the acceleration itself. In most
Since the loudspeakers are in such close
cases this must be determined from exproximity to the microphones, and a
periment, as we have insufficient knowlsymphony orchestra covers such a large
edge to calculate in advance at what G
area, it is necessary to use a much larger
value a long chain molecule will he broken
number of microphones than might otherseveral
up, or a crystal broken into
wise be required. Figure 3 shows a typical
fractions. However, many such effects
orchestra and microphone arrangement.
are in the literature. with approximate Sometimes eight, sometimes ten microvalues of the poweìnnd frequency given. phones are connected to the "Music
Q is the ratio of elasticity to losses, or
Mixer." The second twelve- position
resonant lise in a closed chamber with mixer is employed for soloists, choirs.
perfect reflecting walls. This can be very vocalists, or the control of any sound
high, as shown in connection with water. which must lie balanced against the
7' is the time of application. In many
orchestra.
cases, this is less than a tenth of a second.
Mixing Technique
allowing rapid flow of the material
The mixing technique for a multiplethrough the treating chamber.
microphone pick -up of a symphony
Some values given in the literature are orchestra for sound reenforcement is cononly approximate, as there is often great siderably different from that for broaddoubt about the actual power generated, casting. Not only is there a loudspeaker
and more doubt about how much actually feed-hack problem to be considered, but
appeared in the load. One example is there is also the requirement of having
coagulation of one micron dust of high to match the amplified sound against the
concentration -about one watt per cubic original sound. Any tendency toward
foot per minute, at 24 ke. Another ex- over-or under -amplification is immediateample is degassing metal and controlling
ly noticeable when one has the "real
kw per hundred thing" for comparison. Our method is to
R rain size -shout 2 to 5
pounds at 700 ke.
amplify each section sufficiently to secure
Now that ultrasonics is obtaining con- a "pianissimo balance." When the orchessiderable publicity it would seem attrac- tra conductor or his musical assistants
tive for many engineers to at least obtain agree that a satisfactory low-level balance
a nodding acquaintance with the phenom- has been achieved, the attenuator conena involved by doing some work with trols for the individual orchestral section
the cheap and simple units described microphones are set, and attention is
above. It is a rather peculiar form of turned to the "master gain." Various
energy, and well worth studying, es- tests must then be made to establish just
pecially now that we can visualize means how much amplification is necessary to
make this "smallest" sound readily
of generating practically unlimited power
and much higher power densities than audible in all parts of the Bowl. Once
"pianissimi balance" is established, the
ever before.
AUDIO ENGINEERING
FEBRUARY, 1948
orchestra conductor controls the dynamics of the amplified sound as much as
that of the original. There is a difference
of some thirty VU between pianissimo
and quad-forte, and this much undistorted power output must be available in
the channel.
Levels for the vocalists and instrumentalists are established in the same manner.
and care is always taken not to overestimate the sound necessary satisfactorily
to hear "pianissimo."
Classical Recordings
IJrwa J,,,
i
1
mense values in the unit system enough
to forego the convenience of a simple
console model -but to put things together is another and an insuperahle
problem. We non -engineers are a bunch
of incurable Milquetoasts in this respect!
Most people have an unreasonable fear
of radio innards. There is high voltage
about, they know, and things suddenly
go up in smoke, inside radios. On the
other hand, they are unwilling to call in a
ECONOMY
THROUGH
VERSATILITY..
Letters
f,,,,,, pa,,,
However, a motion study will show
that a piston can be depressed much
more quickly, and with considerably less
movement of the hand, in this manner,
when going from a lower manual to a
higher one, or where the alternate hand
is used.
There is, undoubtedly, a certain dignity
attached to reaching under the manual
with one's thumb and depressing a piston.
But it takes t'me and, with the registration changes demanded by modern
music, a few milliseconds mean a lot,
when approaching the beginning of a
new musical phrase.
(3) Amplitude Versus Frequency Tremolo
I
believe Thal Veil will find through
actual experimentation, that the tremolo
on an organ is a frequency vibrato, and
that my treatment of it is correct.
The subject has been much debated
since the days of Helmholtz
I believe it ranks only second to sea ser en
Many men, far more learned than I shall
ever be, have stated flatly that no appreciable pitch variation occurs in an organ
tremolo.
While I was staff organist at WILL,
1.'rbana- Champaign (1932), t he controversy arose among a group of my friends.
To settle the argument., I connected a
microphone to a cathode ray oscillograph, using an external sweep generator,
so t hat t here could be no coupling bcttceen t he signal and t he sweep circuit.
I later found this to be unnecessary.
The microphone was placed in front of
the organ loft, and the sweep generator
adjusted to hold the pattern stationary
at middle "C," with the tremolo off.
Upon turning the tremolo on, the
waveform shifted from side to side along
the time axis with each hat, indicating
a periodic rise and fall in frequency.
We measured this deviation and found
it to extend roughly to three per cent
each side of the nominal frequency.
There was little or no variation in height
of the waveform; thus, no observable
amplitude tremolo.
This was on a clarabella flute stop.
The strings proved to be about the same,
while the reeds showed very little pitch
deviation with about a ten per cent
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Voicing
The term voicing is used in both
senses -to denote the mere balancing of a
rank of pipes in loudness, and to denote
the entire artistic treatment of the rank.
However, I believe the former meaning is
the more frequent one.
Winston Wells
AUDIO ENGINEERING
BURBANK
FOR RELIABLE CAPACITORS
amplitude variation.
During subsequent years, I have repeated the experiment with improved
apparatus, and have taken many measurements on organs, string and woodwind
instruments and the human voice. In
all instances, the vibrato was shown to
be largely due to a fluctuation in pitch.
(4)
Burbank, Calif.
THE
ELECTRO
MOTIVE Mfg. Co. Inc.
Willimantic, Connecticut
FEBRUARY, 1948
www.americanradiohistory.com
AmericanRadioHistory.Com
43
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I. R. E. National
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March 22 -25, 1948
Grand Central Palace
New York City
HEAR - over 100 Technical Papers,
many on audio topics
SEE - - 167 Engineering Exhibits
of Instruments, Components,
Transmitter Equipment
4 Day Registration" $3.00
to non -members of I. R. E.
THE INSTITUTE OF
RADIO ENGINEERS, INC.
East 79th Street
New York 21, N. Y.
I
44
serviceman to install equipment bought
through other channels. The agony of
assembly is usually turned over, with
enormous trepidation and fear of electrocution, to a sympathetic friend (I
have been that friend); but the fear of
wiring failures, of something going wrong
in the home -made connections never
quite goes! So staunchly does the public
trust the sturdy manufacturer! This,
then, is a major disadvantage that keeps
many of these people from even attempting the unit plan of construction.
Suppose then, to come to the point, I
were asked for suggestions as to how an
enterprising manufacturer might meet
the needs of this growing number of
record owners who are unsatisfied with
{lalventional radios and phonographs?
My approach would be something as
follows:
What is needed, as always, is equipment that will give the advantages desired, while avoiding the disadvantages.
A platitude, but a vital one. It seems to
me that in this case the principle of the
separate unit system is an enormous
advantage. A contrary advantage of
another sort is that of the simple console, both due to its lack of clutteriness
and to the simplicity of its workings.
We must meet squarely the disadvantage
of the hooking-up process in the unit
system. But we must at all costs avoid
the insurmountable disadvantage in the
usual machine of parts inextricably (for
the amateur) intertwined, both electrically and mechanically. Finally we must
meet the cost problem by remaining
strictly in the Gulf area. Our simplest
basic equipment should not cost much
snore thais $75, perhaps $100. Top,
with all the extras should be $300 -$400;
above that we reach the engineer's
stage, already beautifully covered.
The problem can be met, it seems to me,
in the way that the vacuum cleaner men,
the makers of home movie equipment
and medium priced still cameras have
met a similar kind of problem. By
facing the necessity for flexibility, for
complications; and by solving these complications with fool-proof, mistake-proof
instantaneous connections and couplings,
interchangeable parts, ingeniously simplified design that accords with the
modern home owners' idea of convenience
and dependability, that builds confidence
instead of fear. A vacuum cleaner is no
simple instrument these days and a good
camera even less so. But ingenious
(not costly) design has removed the disadvantages to the point where just about
anybody can and does use both.
More specifically? Let me say at
once that what I have in mind would
call for daring and even radical planning,
not as to components, but as to their
housing and the means of connecting
and assembling them, which is the vital
point. Take one aspect only, to begin
with, the housing or cabinet. I suggest
a type of interchangeable -section cabinet.
perhaps two or three basic, simple units,
into which all components could be
fitted, with simple one -twist lock -in
of
mechanical connections instead
elaborate nuts and bolts (elaborate, for
the user). Moreover, these units themselves should be arranged to couple
easily together (in the manner of the
multiple unit p.a. systems now available) to make one solid piece, when
desired. In this way a separate -unit
RECORD LIBRARY
In this spot a continuing list of records
of interest will be presented. The list
specifically does not. suggest "the" Ittt<I
recordings or versions. It will dra\c
predomimantly but not entirely from
postwar releases. All records are theoretically available, directly or on order:
if trouble is experienced in finding them
Audio Engineering will be glad to cooperate. Records are recommended on
a composite of musical values, performance, engineering; sometimes one, sometimes another predominates but records
unusually lacking in any of the three
Number of
will not be considered.
records in album is in parenthesis. C:
other
companies
V:
Victor,
Columbia,
written nitt.
A GROUP OF POSTWAR RECORDS-"B" COMPOSERS.
Beethoven, Symphony No. 6.
Walter, Phila. Orch.
C
631 (4)
B o. 2.
Stern, Zakin.
Beethoven,
C MM 604 (4)
Quartet op. 59, No.
1
"Rasou movsky ").
Paganini Quartet.
(
V M 1151 (Si
Beethoven, Theme and Variations
in F, op. 34.
I.i. il:ud Shure, pianist
Vox 602 (2)
Bernstein, "Facsimile ". (Ballet
music.)
RCA Victor Orchestra, Bernstein.
M 1142
,
2
artok, Violin Concerto. (1941).
\Icnullio, Dallas Symph. Uurati .....
B
.
M 1120 (5
Berg, "Wozzeck" excerpts. {opera
pIi. of Los \ii_.
Is
t'licuiutti
I;u, riser.
Artist JS 12 2 pl.)
Britten, Serenade for Tenor, Horn
and Strings.
l'cter {'ears, Dennis Brain. Boyd Neel
String Orch. Britten.
B ach,
Best" (No.
I
Decca EDA 7
21
Cantata, "God's Time is the
106).
loi yard. Glee Club, Radcliffe Choral
Society, instrs. from Boston Symphony
Technichord T -6 (3 pl.)
B ach, Brandenburg Concerto No.2.
Koussevitsky
Boston Symphony,
(Tanglewood).
M 1118 (4)
(Also includes No. 5).
AUDIO ENGINEERING
www.americanradiohistory.com
AmericanRadioHistory.Com
MM
Beethoven, Violin Sonata, op. 30,
FEBRUARY, 1948
system may have all the advantages of a
console. In fact -beginning from the
other end -we might better call this a
console model with detachable parts,
and make it available primarily in the
form of a number of "console" models, all
assembled from the basic ingredients . . .
Thus nicely having our cake and eating
it too.
It may be asked at this point why such
a scheme should be of interest to engineer
readers of this magazine? Because there
can be no engineer who is not interested
in the wider acceptance of the engineer's
viewpoint towards good sound equipment. Good in quality, but also good in
its ability to meet varying conditions.
It is to his advantage that more and more
people, intelligent, curious people, if
untrained in engineering, see the phonograph not as a mere push -button box
with mysterious dangerous insides but,
realistically, for what it is, an assembled
collection of units basically simple in
their separate functions and, given a
little leeway, highly flexible in their
practical use in the home. Most people
aren't dumb. Lots are intensely curious.
Plenty
big plenty-are ready for a
more complex machine, provided it's
safe, quick to assemble and adjust, absolutely foolproof and above all, with no
menacing "live" wires to touch! Given
this, they'll take almost anything. Look
at the vacuum cleaner. (More next
New Type 1250
High r. f. current carrying capacity
50 amps. max. intermittent load; 30
amps. steady load. Low loss factor.
Sturdy mechanical design ...
Mycalex insulation.
Furnished in any
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-a
Write for Bulletin No. 472
Manufacturers of Precision Electrical Resistance Instruments
337 CENTRAL AVE.
JERSEY CITY, N.
J.
month.)
RECENT RECORDINGS
Respighi, Roman Festivals. (1929). Phila.kip
orchestra, Ormandy.
Columbia MM 707
This is relatively little known sequel to
the brilliantly orchestrated Fountains of
Rome (1916) and the Pines of Rome
(1924). Its lush stuff, fine for hi -fi
recording, but also considerably dissonant, showing the influence of the
harsh 1920s. Recording is wider range,
a bit pinched acoustically. This one
seems to have low turnover point, in
European style (300?). Try it.
Schubert, "Unfinished" Symphony, Philadelphia Otchestra, Bruno Walter. (Comparison: The sauce, with Vienna Philharmonic, Bruno d'alter
.................
Report
\o.
1
from lvpical PARA -FLUX REPRODUCF,R USERS:
BROADCASTING ENGINEER REPORTS:
"Acetate Master Recording plays 295 times
from a PARA -FLUX REPRODUCER
without material wear"
Protecting precious records today is vitally
important. Recordings must last as long
as possible. The above report is a typical
result of the minimum wear on records
.
Columbia MM 699 (Victor G -9)
The latest of dozens, this recording musically is first rate. Technically it is wide
range, but in Schubert this counts for not
much; the music isn't highly overtoned
(as Respighi, above.) Here, the Columbia recording technique, without excessive liveness, is not particularly well suited
to the music. The old Victor album G -9
(out of print) sounds better. It is very
live, and the liveness suits Schubert. Its
range is restricted but an apparent boost
in low highs gives fine feeling of brilliance.
Microphoning, for liveness and detail
too, is absolutely first rate.
Handel, arr. Sir Hamilton Harty, Water
Music Suite. London Philharmonic, Basil
Cameron. (Comparison: The same. London Philharmonic, Harty.)
English Decea EDA 38(Columbia X13)
The Old Columbia recording of this suite
as arranged by Harty (the original has
over 20 movements) has long been a
recorded classic, and was one of the mile -
..............
AUDIO ENGINEERING
Para -Flux Reproducer with
terchangeable Heads:
Universal
Lateral only.
Verticol only
.
in
.
.
.
when PARA -FLUX Reproducers are used.
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Sold through local jobber.
Export: Rocke International Corporation,
.
Write
13 East
For
Bulletin PR4.
40th Street, New York 16, N. Y.
RADIO-MUSIC CORPORATION
PORT CHESTER
FEBRUARY, 1948
www.americanradiohistory.com
AmericanRadioHistory.Com
NEW YORK
45
-
stones in the recording art. It still sells.
The new recording, same orchestra
Harty is dead -is the first since. (There
are other arrangements of the music,
however; none is as good as this one.)
In spite of ffrr, I prefer the old set. No
highs to it, but excellent, highly natural
resonance, enough brilliance to carry the
music. Musically the performance is
superb. The new set is of course wider
range, but the usual ffrr liveness is not
good in this music. The performance
is duller, less precise, more mannered.
One more example of t he fact that wider
range, distortion-free recording is only
the beginning of success in this business!
"Listen to Our Story;" "Mountain
Frolic." American IaIla,Is,'square dances,
hoedowns.
Various
(Records
artists.
originally made in the later 1920s.) ........
Brunswick 1024
1025
Two rather astonishing albums, edited by
Alan Lomax, leading authority on American folk song. These records were all
made before 1930, a number in 1927, and
originally sold as singles. In these repressings they are as remarkable for
quality as many of the oldest hot jazz
records are. No highs, of course. But
otherwise t hey are excellent, well balanced, with realistic and lively acoustics
(no dead studio effects) seemingly very
little distortion, except, as often happens,
assorted blasting due to untrained performers with inadequate mike experience.
The modern repressing gives smooth, if
not silent surfaces. Music is "authentic,"
untouched by radio and commercial
singing; it, is nasal, out of tune, but often
highly musical and sometimes extremely
funny. Try "The Derby Ram!" This
is priceless historical material, as well as
important music -regardless of what, you
call it. (Apologies to Stanleigh, if this
rates as "popular! ")
Disc "Ethnic Series:" (5 albums) Cuban
Cult Music, Folk Music of The Central
East, Haiti, Ethiopia, American Indian
Songs and Dances. Recorded "on location' by Charles Hofmann. Harold Courlander, others ..Disc 131, 132, 141, 142, 161
An astonishing series, representing an
enormous amount of field work with
portable equipment. When you hear the
music" you ll sympathize with the recording personnel! Some of these have
remarkably good quality, though minus
highs. Others are highly distorted,
mainly due to unpredictable noises of
people in violent motion, assorted drums
with superabundance of low frequency
DBs, etc. Try the Central East, Ethiopian albums for best listening, and to
shock your friends and relatives. But
this stuff is to be taken seriously too.
Main trouble: excerpts are too many, far
too short. They fade in, fade out just as
things get going. Excellent booklets, well
illu rä16 d line photo,.
Songs of Tchaikowsky. Irra Pet .ina,
Orchestra conducted by \Valt.t 1Lm11
Columbia MM 712
Technically a very beautiful album.
Singer, tinging in Russian, is close, but
with very great "presence," aided by
hi -fi sibilants, vocal-color overtones.
Orchestra at slightly greater apparent
distance -but by no means a mere background; its tone is live, warm, enveloping.
In short, a perfect recording-for this
type of music. These songs are mostly
piano, originally. Arrangement for orchestra popularizes the album, but does
no great harm, in this case, to Tchaikowsky. Petina has strange mannerisms
(off pitch metes, sli.lings, etc.) but is
t
heart felt, honest singer of her native
musi',.
Saint- Saens, Violin Concerto #3. Louis
Kaufman, Santa Monica Symphony, I {ach-
.....................
Disc. 805
milovitch
Another of the film-music players' orchestra recordings. Kaufman has apparently made a long series of concertos, this
being the 2nd released. Same old story:
poor liveness, dead studio effect. Violin,
beautifully recorded, is too close, balance
is very poor: orchestra far off in the hack ground. Let's hope that before the
December 31 deadline the Santa M.
engineers woke up to the facts (a) that
music must have liveness, and (b) that a
violin concerto is not a violin solo with
faint orchestral background! Most recording companies left that stage behind
ten years ago.
New Products
'from
page .i
pany will also service Vocal -Aire units now
in use.
A new application for the system has been
found in laboratories where studies and
analyses of vitration are conducted. The
high power level of t he modulated air
stream in conjunction with the high frequency range makes it especially valuable
for this purpose.
Inquiries relative to the Vocal -Aire system should be addressed to the new coin-
pany at Seymour, Conn.
It's Tops !
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addition, each month Audio Engineering presents latest improvements in sound reproducing equipment design, test methods, and
The editorial staff includes
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In
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46
AUDIO ENGINEERING FEBRUARY, 1948
www.americanradiohistory.com
AmericanRadioHistory.Com
Hearing Aid Gain
fro
rt
.
page.'
applied to the microphone of the hearing
aid. In the free -field procedure in cornmon use, a pure -tone signal from an
electronic oscillator drives a loudspeaker.
The hearing aid is suspended facing the
speaker so that it receives sound radiated
from the speaker.
Because the distribution of sound radiation from a speaker is non- uniform,
that is, it possesses a directional pattern
which varies strongly with frequency,
it is not feasible to compute the sound
pressure incident on the hearing aid microphone. It is therefore necessary to
measure the sound radiated by the
speaker at the point in space occupied by
the hearing aid microphone. For this
purpose, a calibrated microphone is substituted at the position to be occupied by
the hearing aid. The sound pressure in(heated at each frequency by the measuring microphone is taken to be the sound
pressure incident at the face of the hear ing-aid microphone.
Test Conditions
.
For these measurements to be valid,
sound must come only from the speaker
with no spurious effects introduced by
reflection of sound from the walls, floor,
or ceiling of the test room. The interior
surfaces of the test chamber must therefore be built of material having a very
low reflection coefficient for sound waves.
Moreover, the volume of the room must
be large to minimize reflection, and
sounds originating outside the test chamber must be excluded. The construction
of an " anechoic" chamber with highly
absorbent walls and a sound-isolating
structure is expensive. Acoustic materials
exhibiting low reflection and high sound
absorption are fragile and delicate and
must be carefully protected. A good
chamber can be constructed at an expenditure of the order of a hundred
thousand dollars, though measurements
can be made over a limited frequency
range with a less effective anechoic chamher built at a cost of perhaps twenty
thousand dollars.
Although the Bureau has an anechoic
chamber of adequate quality, it was
necessary to devise some means for testing hearing aids which would release the
test room for other work. The new procedure utilizes a system analogous to a
piston and cylinder for applying the incident sound pressures to the hearing -aid
microphone. The face of the hearing aid
and a calibrated measuring microphone
are mle, in effect, part of the cylinder
wall. They are mounted as flush closures
on opposite sides of a small cavity cut
into a heavy brass block. The driver unit
of a loudspeaker serves as the piston generating the pressure alternations of a
sound signal in the air volume enclosed
by the source cavity. It is coupled to the
source cavity via a brass tube. For sound
frequencies low enough for the pressure
alternations in the cavity to he instantaneously uniform throughout the cavity
volume, the indications of the measuring
microphone are a direct measure of the
sound levels applied to the hearing-aid
microphone. The frequency range is extended to higher frequencies, at which
the sound pressure in the cavity is not
altogether uniform, by mounting the
measuring microphone and the hearing
aid microphone symmetrically with respect to the speaker tube. (Figures1 and2.)
The output sound level produced in
the ear of the user by the receiver of the
hearing aid is measured by the same
technique in both the old and the new
procedures. Ears differ in size and shape,
and so the load which they present to the
receiver differs from one individual to the
next. However, an "artificial ear" has
been devised2 which contains a measur-
TOP NOTCH FOR
'
TOP
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!he new
UNIVERSITY
TWEETER
it with any
Use
Standard Cone
Speaker
So you want high -fidelity? Then this
tweeter is for you. A simple high -pass
filter permits quick connection to your
present cone speaker with only two
wires. Extends the range of your
existing cone speaker to 15,000 cycles.
Available in several types unmounted
or in cabinet. Prices from $20.00. For
details address UNIVERSITY LOUDSPEAKERS, INC.. 80 South Kensieo
Avent. White Plains, New York.
t "Method for Measuring the Perform:ul,,
of Hearing Aids," by Frank F. Romantm
Jour. Acous.Soc. Am. Vol. 13, p. 204 (1912
Fig. 2. Dimensional drawing of the cavity and schematic diagram of
the equipment set up
which utilizes a cavity pressure method fo r measuring the gain of hearing aids.
THREADED
TO FIT
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SPEAKER
NEW!C- MPIETE!
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of
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HEARING
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AID
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DIAGRAM
AUDIO ENGINEERING
OF
SETUP
2CC CAVITY/
CONDENSER
MICROPHONES
FEBRUARY, 1948
www.americanradiohistory.com
AmericanRadioHistory.Com
RADIO
TELEVISION and
ELECTRONIC EQUIPMENT
at LOWEST PRICES
Write for FREE ropy. Address Dept. F -28
CONCORD
CHICAGO
CORP.
7
Ho W. Jackson Blvd.
Downtown ChIcado Branch
3
265 Peachtree St.
225 West Madison St.
LAFAYETTE RADIO CORP.
47
ing microphone in a position corresponding to the tympanum of the human ear,
and which presents to the receiver of the
hearing aid a load respresentative of a
human ear fitted with an ear mold. Comparison with measurements on human
subjects has shown that this "artificial
ear" provides a valid though rather inexact means for measuring the output of
the hearing aid receiver. The differences
between the results obtained by using the
artificial ear and those obtained with
human subjects are sufficiently small so
that they do not mar the significance of
data obtained using objective method.
The equipment required in the cavity
et
heat
oca4
E
Y
oIr
Cje
le.t
cY,ao
Eg
aN
IT
Cook Exclusive Q-C Quality Control)
recording doe; these things.
db Better Signal- Noise.
All Masters Uniformly
Distinguished in quality.
Full Frequency Range
Perfectly Clean.
-
l.normous Volume Level
Double the
Capacity
Dynamic Range without
Increased Record Wear.
-
We are equipped to do your Re- Recording, or to supply complete Q -C Recording
Systems.
Write for Detailed Information.
COOK LABORATORIES
139
Gordon Blvd.
Floral Park, N. Y.
AUDIO ENGINEERING SCHOOL
months' course in Sound Fundamentals, Recording, and Sound Transmission
measurements; in a laboratory containing transmission sets, oscillators, square wave generator
and intermodulation analyser, and other equipment.
Complete recording studios assimilating broadcast, motion picture and commercial sound recording.
A practical 9
NO
For information write
H. M. Tremolos, Director
SCHOOL
ION ENGINEERING
TR
TRANSM ISSION
Hollywood 28, Calif.
1665 -A Cherokee
VERTICAL
LATERAL
9A, 9B, D- 93306, MI -4856
Reproducers Reconditioned
Only new parts used -includes repainting and
replating:
Response checked with advanced Laboratory
equipment
FAST SERVICE
VIBRATION SYSTEMS, INC.
1040 W. Fort
48
Pickup Technique
[front page
14J
line and indicate the results that might
be expected using the various pick -ups,
microphones, and acoustical conditions.
They are intended as a guide rather than
strictly formulated rules. There is no
known substitute yet for individual
judgment, taste, or listeners' reactions,
which are the principal guides in achieving the optimum results. The success of
every broadcast depends on sound fundamental principles intelligently applied
with flexibility and originality, taking
full advantage of every technical advance to meet the needs of a particular
situation. In this way, only, can the skill
in technique of broadcasting keep pace
with the engineering developments so
frequently being provided.
The author wishes to acknowledge the
generous help of the NBC staff and is
Loud Speakers
particularly indebted to Rinehart
[from pays 2,
and Company for permission to reproextravagant rating should a competitor duce Figs. 9 to 15 from their current book
"Broadcasting Music" by E. La Prade
care to use it.
Efficiency rating is usually avoided by (1947).
the manufacturer and it has become common to judge the efficiency of a speaker by
the size of its magnet. There is even a
ADVERTISING INDEX
general impression that the larger the
The
outit
is.
speaker, the more efficient
2
Arnold Engineering Co.
put of a speaker is proportional to the
6
Audio Development Co
flux density in the air gap, the length of
Audio Devices, Inc..... Cover 2
wire in the voice coil and the current in
Audio Engineering School ... 48
the voice coil. The size of the speaker or
39
Audio Equipment Sales
magnet has nothing to do with efficiency
except as it may help determine one of
Bardwell & McAlister, Inc . 41
the three critical factors. The apparent
Brociner Electronics Labora40
loudness of two speakers is not an index
tory
7
of efficiency either, since loudness depends
Brush Development Co.
also on directional characteristics and
5
Camera Equipment Co.
frequency response.
Chicago Transformer
of
capacity
-handling
the
power
Rating
Cover 4
Corp.
a speaker is extremely dangerous as well
43
Cinema Engineering Co...
as difficult. In the first place, what does
47
Concord Radio Corp.....
it mean to specify that a speaker is a
48
Cook Laboratories
25 -watt speaker? It has been found1,2
Mfg.
Co.,
Motive
Electro
that the long time average power for
43
Inc.
speech is 20 db less than the instantaneous
33
Inc.
Voice,
Electrovalue
is
this
music,
For
peak power.
3
Cover
Co.
Radio
General
speaker
approximately 12 db. Thus, a
42
Harvey Radio Co., Inc.
capable of handling 25 -watt instantaneous
Institute of Radio Engineers. 44
peaks need only handle 1.6 watts of con36
LeBel, C. J.
tinuous power. For this reason, a speaker
36
rated at 25 watts generally will not
professional Directory
handle a continuous 25 -watt sine wave
1
Racon Electric Co., Inc.
45
at all frequencies in its range without
Radio Music Corp...
danger of mechanical damage or burn37
Shure Brothers, Inc..
out. More often, however, a speaker's
45
..
Laboratories
Tech.
power handling capacity is limited by
41
Terminal Radio Corp.
distortion.
36
U. S. Recording Co..
' H. K. Dunn and S. D. White "Statistical
44
Universal Microphone Co.
Measurements on Conversational Speech,"
University Loudspeakers, Inc. 47
Jour. Acous. Soc. Am., Vol. 11, Pt. 278;
48
Jan., 1940.
Vibration Systems, Inc..
s L. J. Sivian, H. K. Dunn and S. D. White
36
Winston
Wells,
"Absolute Amplitudes anti Spectra of CerWestern Electric Co....... .. 8
tain Musical Instruments and Orchestras,"
38
Wrigley, Wm. Jr. Co.
Jour. Acous. Soc. Amer., Vol. 9, pp. 1 -10;
July, 1937.
AUDIO ENGINEERING FEBRUARY, 1948
1
I
I
,
Q -C Recording ACTUALLY DOES
IO
pressure method for applying known
sound levels to the microphone of a hearing aid may be simply constructed in a
machine shop. The entire source cavity
unit, including the calibrated standard
microphone, its associated amplifier and
output meter, the speaker unit, and signal
oscillator can probably be assembled at
an expenditure of a few hundred dollars.
The cost of a complete "artificial ear"
including the measuring microphone and
its associated amplifier and output meter,
and a small brass block of which the "ear
canal" is constructed, is of the order of
several hundred dollars.
The construction and maintenance of
a specially- treated echoless room is expensive. The simpler test procedure
developed at the Bureau provides a more
economical means of testing hearing aids,
and one that can easily be adopted by
any manufacturer.
Detroit 26, Mich.
.....
Iti
NEW AUDIO
with
LOW DISTORTION
SUPERSONIC OSCILLATOR
&
HIGH STABILITY
UNIFORM OUTPUT
This new wide- range, continuously adjustable oscillator was designed to
fill a need for an instrument for laboratory measurements of gain, distortion,
impedance and frequency response at frequencies well above the audio range.
With a single dial and four push- button-controlled multipliers, the Type
1302-A Oscillator covers the range of 10 to 100,000 cycles. Because of its wide
frequency range and flat output this oscillator is particularly suited to taking
frequency response characteristics on amplifiers, telephone lines, filters and
other such circuit elements.
FEATURES
RANGE -10 to 100,000 cycles -180-degree rotation
of dial
WIDE FREQUENCY
covers the 10 to 100-cycle decade -panel push buttons add in decade steps
ACCURATE CALIBRATION -adjusted with +(1 -)z' ó +0.2 cycle)
LOW DISTORTION -less than 1' ;, at any frequency
SMALL FREQUENCY DRIFT -less than l' , in first 10 minutes; less than '0.2' ;,
per hour afterwards
FREQUENCY DRIFT CONSTANT PERCENTAGE OF OPERATING FREQUENCY- particularly helpful with bridge measurements at low frequencies
CONSTANT OUTPUT VOLTAGE -within ± 1.0 db over whole range: 20 volts
open circuit on 5,000 -ohm output, 10 volts on 600 ohm
STABILIZED SUPPLY-compensated for transient line voltage surges and average line voltage variations between 105 and 125 (210 and 250) volts
VARIABLE CONDENSER FREQUENCY CONTROL -avoiding contact difficulties often found in variable resistance control
TWO SEPARATE OUTPUT CIRCUITS-balanced 600 ohm and unbalanced
5,000 ohm
TYPE 1302 -A
OSCILLATOR
S365.00
GENERAL RADIO COMPANY
90 West
St., New York
6
920 S.
Michigan Ave., Chicago
5
www.americanradiohistory.com
AmericanRadioHistory.Com
950 N.
Cambridge 39,
Massachusetts
Highland Ave., Los Angeles
38
AUDIO TRAP SFOR
UL
FREQUENCY RANGES
IN
if ; rei /t7/o7
iltoreiny cCiíi/e/.
11,1
1
ERS
J/iei
Full Frequency Range
30 to 15,000 Cycles, provides uniform response over this entire band with
1/2 db up to 10 watts of audio power,
within ± 1 db over 10 watts. Standard
RMA impedances. Hum balancing coil
structures and nickel alloy shielding.
Included are Input, Output, Driver, and
Modulation Transformers; Modulation
Reactors. Sealed in Steel construction,
stud mounting, with pin -type terminals.
Public Address Range
y
50 to 10,000 Cycles, frequency response within ± t/z db up to 10 watts
of power, within ± 1db over 10 watts,
throughout this range. Secondary impedances match 600 and 150 -ohm lines,
16, 8 and 4 -ohm reproducing systems.
Listed are Driver and Output Transformers. Sealed in Steel construction,
flange mounting, with solder lugs or
wire leads.
i
Communications Range
200 to 3,500 Cycles, affords response
with variations not exceeding ± 1 db
over the range of voice frequencies. For
use with 600 or 150 -ohm lines. Input,
Output, Driver and Modulation Transformers offered. Sealed in Steel construction, flange mounting, with wire
leads or solder lugs.
DIVISION OF ESSEX WIRE CORPORATION
3501
A D D
I
S
O N
S T R E E T
C H
www.americanradiohistory.com
AmericanRadioHistory.Com
!
C A G
O
18,
I
L
L
I
N O
I
S
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