JBL - diyAudio.rs
 JBL Signature Koustical Lens assembly 175D LH
made up of a high frequency driver, exponential horn,
and diffusion lens which radiates undistorted highs
evenly over a solid 90° angle. The Koustical Lens
operates on principles never before incorporated in
a manufactured unit and made available to the audio-
signature phile. It is not to be confused with the ordinary
“tweeter, Nominal impedance is 16 ohms; power
input, 25 watts above 1200 c.p.s.; field 18 а permanent
magnet; index of refraction is 1.3; diameter of horn
throat 18 17; mounting hole diameter 5%"; overall
length 11%"; shipping weight 12 pounds.
Dividing network N1200, a 3 position high fre-
quency attenuation switch with a nominal impedance
of 16 ohms, crossover at 1200 ¢.p.s. Has attenuation
of 12 db per octave.
15" low frequency speaker No. 130A. The 4” voice
coll of this unit assures smooth, clean lows of
unmatched fidelity, Nominal impedance, 16 ohms;
power input, 26 watts; permanent magnet field;
resonant frequency, 36 cycles; outside diameter
| 15-13/16 ; depth 6% ; shipping weight, 23 pounds.
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JU] ONNOS INISNYT E SIiWYI
MODEL 37 MODEL 35 Console
Ld Boy
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General purpose speakers |
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12” D131 15 D130. Rated at the top by many independent agen-
| cies for its crisp response across the entire audio
frequency band, A 4"-diameter voice coil of
edge-wound aluminum ribbon is subjected to intense
magnetic flux, which is efficiently conducted from a
permanent magnet. Power input, 25 watts; imped-
ance, 16 ohms, Dimensions; 15%," OD x 5%" depth.
Weight: 23 lbs.
12” 0131. The same quality —the same design and
construction as Model D130. Differs only in the
smaller dimensions of frame and cone. The finest
12” speaker made anywhere.
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12” 0123. Amazing fidelity from a 12" speaker of new 15” D130 A
shallow design. Features a 3"-diameter voice coil of
edgewound aluminum ribbon. Power input, 20 watts; |
impedance, 16 ohms,
© 12" D123 |
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High Frequency Ring Radiator 075
| MODEL 39 Multiple/Reflex MODEL 38 Ló Boy
The newest member of a distinguished family,
JAMES B. LANSING SOUND, INC. this extremely efficient transducer adds brilliance and
dimensionality to the performance of general pur-
pose speakers and is particularly recommended for
Los Angeles 39, Calif. use in the more absorbent home rooms, where the |
highs are selectively attenuated, Radical new princi- d a
ple permits the closest possible approach to piston \ Nr
action from a 2500 eps crossover to a point beyond wy AF
the limit of human audibility. N2600 Dividing Net- |
work has built-in HF volume control, so that 075 |
can be balanced with single or multiple general- "Le
purpose speaker installations,
3249 Casitas Ave.
signature
models C36, C38
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“Do-it-voursell'' legs for
| Model C36 Enclosure.
| Cut four; mount diagonally.
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These minimum-volume reflex
enclosures will accommodate
a variety of loudspeaker
components. Model C36 is an
upright enclosure which uses
the diagonal legs shown here.
Model C38 1s a lowboy version
of the same enclosure which
uses tapered wooden legs.
Two of these reflex units with
appropriate loudspeakers
installed make an excellent
space-saving stereo
speaker installation.
Detailed blueprints and assembly instructions
for the C36 and C38 are available at nominal
cost through vour authorized JBL dealer.
models C35, C37
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The C37 lowboy and C35 console enclosures
are shown in the photographs on the
cover of this publication. As indicated by
the dotted circles on the baffle layouts
above, the high frequency mounting hole
may be located on either side of the
cone speaker. When building two matching
enclosures for a stereo system, the high
frequency transducer can be mounted to
the right in the right-hand cabinet, and
to the left in the left-hand cabinet,
In this way, optimum separation of sound
sources can be achieved with close
spacing of the enclosures.
Detailed blueprints and assembly instructions
for the C35 and C37 are available at nominal
cost through your authorized JBL dealer.
7
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model C39
В : _ | ~ general construction notes
1. %" plywood is used for all panels. First assemble the top, bottom, and sides.
Then fit the scabs inside the front opening. Finish the exposed edge with
molding, veneer, or lacquer.
Detailed duplication of the exterior
stvling which characterizes this
sturdy corner enclosure 18 a project
recommended only to experienced
2. Cut the speaker baffle for the loudspeakers which will be installed.
If you plan to add a high frequency unit later, cover the unused hole with
a plywood block screwed down tight. Stretch and tack grille cloth over the
front of the baffle after painting it black. Fasten the completed baffle to the
inside edge of scabs with wood screws every four inches.
woodworkers. The bevel cuts which
give the “sculptured” effect to
the top and bottom panels require
a duralloy blade operated with
considerable skill. Veneer strips are
glued to the edge surfaces with
grain perpendicular to the long edge
lines. The baffle shown is one of
3. Acoustical Fiberglass padding one-inch thick is tacked inside back,
bottom, and sides of the enclosure.
4. JBL dividing networks can be mounted on the back of the enclosure in
two designs adopted to accommodate a position which won't interfere with loudspeaker placement. A hole
the numerous speaker systems 4" x hr” should be Cut in the back panel. If the JBL N 1200 is used,
which can be used in this enclosure. it 18 mounted inside the hole on two blocks 4%" x 7" x %”.
The second design is for 12-inch
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es p to four « diameters [or JBL cone loudspeakers Thy ete I и
Зе > | and high frequency transducers.
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Detailed blueprints and assembly instructions INTERNAL VOLUME OF CABINET IN CUBIC FEET
for the C39 are available at nominal cost If you make a rellex enclosure in which to mount JBI, speakers
with dimensions other than those shown here, the correct port size
through your authorized JBL dealer, | can be found on the above chart after you have calculated the cubic content.
BASS RE XES ___——
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A Loudspeaker Needs an Enclosure
If you lay a loudspeaker on a table
and feed a low frequency signal into it,
you will see the cone vibrate but will hear
no sound.
There are two reasons why this is so:
1. At very low frequencies, the cone is
not large enough in diameter to couple its
energy to the air.
2. All sound that is generated by a
loudspeaker is produced with equal inten-
sity by both sides of the cone. When one-
half the length between sound waves is
approximately equal to the diameter of
the speaker, radiations from the rear side
of the cone combine with radiations from
the front side of the cone. Since they are
out of phase, they tend to cancel each
other.
The first problem (coupling) can be
solved in several ways, all of which have
their advantages and disadvantages. The
cone can be made larger in diameter.
Beyond a certain point, however, the dis-
advantages outweigh the advantages.
The effective diameter of the “piston area”
may be increased by using two or more
speakers to reproduce exactly the same
frequencies. This is a common solution,
but is expensive because multiple low-
quality speakers will not solve the prob-
lem; they are still subject to many of the
same shortcomings that are apparent when
used as single units. Another approach
would be to make the cone move back
and forth (linear excursion) a greater
distance. Here again, the problems intro-
duced offset the gains made. As evidence
of this, there is no unit commercially
available to adequately satisfy this coup-
ling requirement. The speaker may be
mounted in a horn. A properly designed
horn operates in the same manner as the
flared horn used in musical instruments —
a relatively small sound source being
coupled to the large air mass in the bell.
One objection to a horn is that it is difficult
and expensive to build,
The second problem mentioned above
— cancellation —may be solved in several
ways. The speaker may be mounted in a
large, flat, “infinite” baffle so that back
radiations are physically prevented from
combining with front radiations. Speakers
are sometimes mounted in the wall of a
room. This eliminates the cancellation 4
effect; but low-end reproduction is in gen- —
eral far from satisfactory. Mounting a
speaker in a completely airtight cabinet
of adequate volume which is lined with an
absorbent material accomplishes the same
result; but adequate low-end response is
still lacking. Mounting a speaker in a
totally enclosed cabinet of inadequate vol-
ume merely adds an additional bad fea:
ture. The stiffness of the sealed-in air
drastically affects the cone movement, and
at the resonant point produces a bump in
the response. Contrary to widely-pub-
lished opinions, most modern efficient
loudspeakers are not designed to operate
well in an infinite baffle installation.
Theory of Bass Reflex...
The bass reflex cabinet, sometimes
called a “phase inverter? is a practical
solution to both the problems mentioned
at the beginning of this discussion. It is a
strong, large cabinet with airtight joints,
and a baffle for mounting the speaker or
speakers. A port—a hole of carefully cal-
culated dimensions — is cut into the baffle
close to the speaker. The energy produced
by the back side of the cone excites the
air contained within the cabinet, By basic
physical principles, the air so excited
within the cabinet moves in and out of the
port in exact phase with the sound gene-
rated by the front side of the cone. By
correctly designing the reflex enclosure,
movement of air in and out of the port can
be made to take place at a point where
the loudspeaker, without help, is incapable
of coupling its energy to the air.
The reflex cabinet does three things.
ee
First, it prevents radiations from the rear
of the cone from canceling those produced
on the front. Secondly, radiations from
the port, since they are in phase with front
radiations, reinforce the low notes by pro-
viding a second sound source. Thirdly,
when the loudspeaker begins to cause air
to move in and out of the port, it is being
forced to do work. This loads the speaker
far better than does the infinite baffle.
Because of this loading the speaker cone
is generally moving no further back and
forth at 40 cycles than it is at 150 cycles.
In an infinite baffle the cone must move
many times as far at 40 as it does at 150
cycles in order to produce tones of the
same loudness. Low note fun i
are reproduced at much greate volume
and with far less distortion in a proper! y
designed reflex enclosure than Ue are
with the same speaker mounted in an
infinite baffle.
It is impossible to give spa dimen-
sions and instructions for building the
proper reflex enclosure for all types and
makes of cone loudspeakers. Enclosures
described here are designed for use with
units manufactured by James B, Lansing
Sound Ine. For other loudspeaker mech-
anisms, the specific advice of the manu-
facturer should be obtained in order to
achieve satisfactory results, Ie
General considerations of
Bass Reflexes...
When balancing results against ease of
construction, it can be seen that the reflex
eñelosure is the type most practical for
building in the home workshop. Though
straightforward in construction, it must
be built well. Dimensions must be reason-
ably aecurate, joints must be true and
tight. Lock-mitre joints, glued under
clamps, are ideal if you have access to
mill-working machinery. Otherwise all
joints should be reinforced with glue
blocks running the entire length of the
joint, Glue blocks should be screwed at
four-inch intervals to each surface.
The interior of the enclosure should be
padded with any soft absorbent material.
It must, however, be 1” in thickness. If
rockwool or a similar material is used, it
should be covered with cloth to prevent
contamination of the loudspeaker itself.
The padding should be applied to two-
thirds of the interior surface. Most satis-
factory performance is generally achieved
by arranging the padding so that a padded
surface faces a bare surface of the other
side of the enclosure. The optimum size
of reflex enclosures for 12 and 15 inch
speakers is between 6 and 12 cubic feet.
For 8 inch speakers, good results may be
obtained with as little as 3% cubic feet,
The most important single factor in
constructing a reflex enclosure is to elimi-
nate all possibility of vibration or move-
ment of the walls of the cabinet itself. In
an experimental enclosure, which differed
from the specific dimensions shown herein,
the best method of achieving this rigidity
is to first construct the cabinet without
attempting to predetermine the interior
bracing that may be necessary.
After the cabinet has been built, mount
the loudspeakers and operate them from
your power amplifier on some program
material having good low notes. The vol-
ume control should be adjusted so as to
be as high, or higher than you would ever
normally use as a listening level. Then run
your hand over the entire outside surface
of the enclosure and note any areas which
move or vibrate in the slightest degree.
You must then install sufficient bracing or
strengthening on the inside of the enclo-
sure, so as to prevent this movement, It
may be necessary to go through this pro-
cedure more than once in order to achieve
satisfactory rigidity. When this has been
done the enclosure will provide its opti-
mum response, and, particularly, response
at a very low level will be benefited by its
rigidity.
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