Jan. 31, 1939.
Filed Aug. 25, 1954
Patented, Jan. 31, 1939’
Marshall L. Lockhart, Des Moines, Iowa
Application August 25,1934, Serial No.- 741,518
5 Claims. (Cl. 179—121)
An object of my present invention is to pro
vide a novel sound pick up and amplifying appa
ratus of simple and inexpensive construction
which is an improvement over similar apparatus
5 shown in my eo-pending application Serial No.
‘709,797, filed February 5, 1934, on an “Electro
More particularly it is my object to provide a
sound pick up and amplifying apparatus having
10 a simple microphone arrangement provided with
means to exclude extraneous sounds, or sounds
other than those to be picked up by contact of
the microphone with animal tissue or' the like,
and which is so connected with a vacuum tube of
15 an ampli?er that tube noises and noises- caused
by making electrical connections at certain usual
points in an ampli?er of the vacuum tube type are
eliminated, whereby the outgoing ampli?ed elec
tric current to an audible translation device or a
20 recording device ?uctuates in almost exact ac
cordance with frequencies picked up by the micro
phone and without imposed frequencies caused by
tube noises and the like.
A further object is to provide a microphone of
'26 the crystal type, such as a piezo cell or crystal
pressure type microphone, which is inherently
supersensitive and capable of producing potential
diiference in a conductor without feeding any ex
' ternal electric current to it, the microphone ele
30 ment being enclosed in a heavy casing which has
a frequency of vibration much higher than any
sound to be picked up by the apparatus and which
includes an air column of minimum dimensions
so that the diaphragm of the microphone is
'35 almost entirely responsive to sounds picked up
without any distortion caused by a column of air
between the object from which the sound is picked
up and the diaphragm.
Still a further object is to provide means for
40 relieving air pressure on the diaphragm when the
microphone is made to contact with animal tissue
or the like which means is not aifected in any
way by air currents outside the microphone cas-’
ing and is not susceptible to variations caused by
45 talking or other sounds outside the microphone
casing, nor is it possible to accidentally prevent
relief of air pressure on the diaphragm, as by an
operator grasping the microphone with a?nger or
thumb over a pressure relief opening'in the eas
50 ing.
Still a further object is to connect the micro
phone with a vacuum tube of an ampli?er in such
manner that tube noises and other undesired
frequencies are suppressed, the manner of con
55 nection being by a shielded lead having its shield
connected with a tube shield, which in turn is
connected with a cathode of the tube and with the
ground in the ampli?er, the lead having its con
ductor connected with the grid of the vacuum
tube and the microphone and a resistance for 5
stabilizing the frequency at which the microphone
operates being shunted across the terminals of
the microphone rather than across the cathode
and grid of the vacuum tube as ordinarily done,
thus preventing, in my apparatus, undesired po- 10
tential difference in the microphone lead, thereby
eliminating any howling or screeching effect in
an audible translation device or undesired fre
quency recording in a recording device connected
with the ampli?er and caused by a potential dif- 15
ference in the microphone lead.
Still a further object is to provide a modi?ed
form of the microphone which is comparatively
small and shaped somewhat like the bell of a
stethoscope, this form of the invention having a 20
corrugated diaphagm as close to the mouth of
the microphone as is feasible for reducing unde
sired effect of an air column between the object;
from which sound is to be picked up and the dia
With these and other objects in view my inven
tion consists‘ in the construction, arrangement
and combination of the various parts of my de
vice, whereby the objects contemplated are at
tained, as hereinafter more fully set forth, pointed ‘80
out in my claims and illustrated invthe accom
panying drawing in
Figure 1 is' a persp ctive view of a microphone
or sound pick up device embodying my invention.
Figure 2 is a sectional view on the line 2-2 of 35
Figure 1.
Figure 3 is a sectional view on the line 3-3 of
Figure 2.
Figure 4 is an enlarged sectional view on the
line 4-4 of Figure 3 showing a speci?c type of 40
lead for the terminals of the microphone.
Figure 5 is an enlarged sectional view on the
line 5-5 of Figure 2 showing how the mouth of
my microphone contacts with animal tissue or
the like to form an effective sound excluding seal 45
Figure 6 is a sectional view similar to Figure 2
showing a modi?ed form of the microphone; and
Figure '7 is an electrodiagrammatic view show
ing the microphone and amplifying apparatus 50
connected with apparatus whereby the picked up
and amplified sounds can be heard and/or re
On the accompanying drawing I have used the
reference character A to indicategenerally the 55 '
terminal 40 of the microphone while the shield
microphone. It comprises an annular casing 10
66 is connected to the terminal 42. A stabiliz
ing resistor 10 is arranged within the microphone
casing to shunt the microphone terminals, it pref
having a front wall l2 and a back wall l4 having
a screw threaded or other suitable connection
with the casing ID.
The mouth piece 25 extends
from the front wall I2 and is of a. bell shape.
erably being connected directly with them so as
to be connected in the circuit as close to the mi
its outer end adjacent its periphery, it is provided
with an annular groove l8. Inside of the groove
crophone as possible.
In Figure '7, I have shown the microphone thus
far described connected with a vacuum tube 12
of an ampli?er. The‘tube is of the triode type, 10
I 8 is a shallow depression 20 which communi
cates with a bore 22.
Within the casing I0 is a disc shaped base 24,
and an annular ring 26 both formed of insulat
ing material and a piezocrystal or other micro
phone element 28. A diaphragm 30 has its pe
ripheral edge contacting with the ring 26 and
15 has on its front face a small porous ring or gas
ket 32. The elements 24, 28, 30 and 32 are _all
sealed relative to each other by glue or the like
and are held as a unit within the casing 10 by a
pad of compressible felt or the like .34 and the
back wall I4, when in assembled position rela
tive to the casing‘ 10.
having a cathode ‘I4, a grid 16 and a plate 18. For
best results the outside of the tube should be
shielded as with a shield 80. The cathode and
shield are ordinarily grounded, although if not
inherent in the tube used, it should be so con 15
nected when the tube is installed in the ampli?er
to obtain best results.
I have found it quite important to directly con
meet the conductor of the microphone lead vB to
- the grid cap 82 and the lead shield 64 with the 20
The microphone element 28 is preferably of
the piezocrystal type now in ‘general use and
consisting of a central conductor plate 38 of tin
foil or the like and an outer conductor plate 38
which extend to terminals 36a and 380, which
in turn are wired to terminals 40' and 42 on the
back of the base 24. The plates 36 and 38 are
separated by crystals 44 of Rochelle salt or the
like and which are susceptible to a bending pres
sure for varying the potential across the termi
nals 40 and 42. Bending due to pressure is made
possible by anchoring the ends of the element
28 to blocks or the like 46 and connecting the
center of the element as by a wire 48 to the cen
ter of the diaphragm 30. v
tube shield 80, whereas ordinarily the conductor
and lead'shield are brought in beneath the shield
ed base of the ampli?er connecting the lead shield
with the shielded base to ground it at~that point
and extending the conductor up-to the grid cap. 25
This produces undesirable noises which are_elim
inated only by a direct drop of the lead B down
to the vacuum tube 12 and connection of the lead
shield with the tube shield rather than with the
ground even though eventually the current from 30
the lead shield reaches the ground, through the >
tube shield and cathode ground wire.
The ampli?er is connected with an ordinary
translation device, such as a loud speaker. or a
series of head phones and with a recording de
vice such as shown in my co-pending application,
by means of wax 50 applied in a melted state after
so that the sounds picked up can be either heard
or recorded, or simultaneously heard and record
the parts are assembled so as to eliminate assem
ed if desired.
The connection at the center is preferably made
40 bly with any strain on the diaphragm and mi
crophone element. As before stated, the ele
ments 24, 2B and 30 are sealed relative to each
other and the wax 50 completes the seal so that
the microphone element is enclosed in a hermet
45 ically sealed container consisting of the dia
phragm 30, the ring 26 and the base 24 so that
moisture cannot enter and deteriorate the crys
tel 44. This element, however, forms no part of
my present invention; I have simply found it the
50 most satisfactory microphone element for the
electrostethograph disclosed in my co-pending
For protecting the diaphragm 30 against in
jury I provide a heavy screen 52 in the bore 22.
55 The casing I 0. if made of metal, shouldbe ground
ed to one of the terminals of the microphone and
I provide for such ground in ‘the following man
The terminal 42 has connected with it a strip
.60 of metal 54 which at its outer end is bent into the
form of a circle as indicated at 56 and is a snug
In the form of invention shown in Figure 6, 40
many of the parts are similar to those shown
in Figure'2 and in such case are given the same
reference numeral with the addition of the dis
tinguishing characteristic.
The depression 20 and bore 22 of Figure 2 are 45
eliminated, the diaphragm 30' of Figure 6 being
located as close to the‘ outer end of the mouth
I 6' as possible to minimize the air column be
tween the object with which the microphone con
tacts and the diaphragm. In Figure 2 it will be 50
noted that the front wall l2 closely follows the
contour of the diaphragm 30 to minimize this
The‘ diaphragm 30' is preferably corrugated so
that disc like portions 30a thereof can vibrate in 55
response to sounds while cylindrical portions 30!)
thereof are not responsive to vibration but re
inforce the diaphragm without detracting from
its vibratable area.
The base 24' for the microphone element 28'
may be secured in position as by screws 25 and
.?t between the periphery of the base 24 and the > a small vent 3| may be provided in the dia-v
phragm 30’ to allow escape of air when the mic
casing II).
A lead B extends into the microphone casing rophone' is brought into contact with animal
65 through an opening 58 in the wall thereof. The tissue or the like.
In the device shown in Figure 2, escape of air
lead has a conductor 60 surrounded by insulation
62 and a current conducting shield 64. Outside from in front of the diaphragm is permitted be
cause of the porosity of the gasket 32 and the
of the lead shield 64 there is a cover 66 prefer
ably of porous material, the purpose of which restricted passage a?orded between the periph
eries of the diaphragm 30 and the base 24, these‘ 70
70 will hereinafter be described. As shown in Fig
parts being a snug ?t but not an airtight ?t.
ure 4, the lead is substantially a snug ?t in the
opening 58.
Within the microphone casing the lead may be
In operation the microphone, which is espe
cially adapted for picking up heartgsounds, may
secured to the base 24 as by a clip 58 and the
75 conductor 60 of the lead is connected with the‘
be adjusted by a physician over the heart or a
patient with the bell mouth I6 in contact with TI
the patient’s chest, indicated at C in Figure 5.
The annular groove I8 provides a double contact
of the mouth at 84 with the patient, which effec
tively excludes sounds because of making an air
tight connection and also by reason of the slight
dead air space formed at 86. I have found that
even when the mouth of the microphone con
tacts with hair on the skin an efficient seal can
be effected so that extraneous sounds are ex
10 cluded and only the sounds coming through the
skin of the patient affect the microphone ele
Minimizing the air column between the patient
and the diaphragm is also quite important as an
excessive air column tends to suppress the sound
waves before they reach the diaphragm, thus
giving a somewhat false and insu?icient vibra
tional magnitude to the diaphragm.
Upon contact of the patient with the micro
phone, air is compressed between the patient and
the diaphragm because of the skin of the patient
bulging into the microphone. This must be
taken into consideration and has heretofore been
taken care of by others by making a vent in the
25 front cover I2.
This has considerable objection
as extraneous noises can be picked up through
this vent while any wind blowing across it will
produce an undesired vibration on the dia
Figure 5 I have omitted any felt packing or the
like as the felt is not entirely necessary, although
it is preferable.
Some changes may be made in the construction
and arrangement of the parts of my device with
out departing from the real spirit and purpose of
my invention and it is my intention to cover by
my claims any modified forms of structure or use
of mechanical equivalents, which may be reason
ably included within their scope.
I claim as myinvention:
1. In a sound pick up apparatus, a microphone
including a mouth extending therefrom for con
tact with animal tissue or the like from which
sound is to be picked up, said mouth adjacent
its outer periphery having an annular groove,
the sides of which contact with said animal tis
sue or the like and provide a dead air space be
tween said sides.
2. A sound pick up device comprising a casing, ,
a vibratory diaphragm therein adjacent the
front thereof, said diaphragm being of corru
gated construction, a crystal type microphone
element connected therewith and mounted in
said casing, a leak from in front of said dia
phragm and it also has the disadvantage of being
phragm to the portion of said casing behind
said diaphragm and a leak to atmosphere from
said portion.
3. In a sound pick up apparatus, a microphone
30 in an exposed position where it can be closed up
including a mouth for contact with an object
by a ?nger or thumb of the operator or become
at its outer end having an annular groove ad- 1
jacent the periphery thereof for forming a double
In my microphone construction, an air cham
ber is provided behind the diaphragm and any
air compressed in front of the diaphragm can
leak in a restricted manner to this air chamber
either past the porous gasket 32 or through the
restricted vent 3! in Figure 6.
This air chamber
can be sealed if found desirable or it can have
40 a slight leak to atmosphere so that the pressure
therein can be equalized with respect to atmos
pheric pressure. Since a lead B is necessary for
the microphone, such a leak is readily provided
through the opening 58 in the casing for the lead.
45 The lead preferably snugly ?ts this opening, while
if the lead is covered with a fabric material such
as shown at 66, the .cover forms an effective dust
screen as well as permitting restricted leakage of
air into or out of the casing l0.
from which sound is to be picked up, said mouth
clogged with dirt, thereby entirely defeating its
As a further precaution against sound enter
ing the casing and affecting the diaphragm 30
the felt 34 is provided, which acts as a sound
deadener and baffling material as well as a ?lter
material to prevent dust entrance. The felt may
55 be in the form of a disc, and this disc may be of
other material than felt, such as cotton or the
rimmed seal with such animal tissue or the like
and for providing a dead air space between the
4. In a sound pick up apparatus, a microphone
including a casing, a mouth thereon for con
tacting with an object from which sound is to
be picked up, a cavity enclosed by said mouth.
a diaphragm within said casing, a microphone 40
element connected therewith, a chamber within
said casing behind said diaphragm, means for
equalizing the pressure in said cavity with the
pressure of the atmosphere when said mouth is
placed in contact with an object, said means in 45
cluding a leak from said cavity to said chamber
and a leak to atmosphere from said chamber,
and sound absorbent material between said leak
to atmosphere and said diaphragm.
5. In a sound pick up device, a casing, a 50
mouth thereon for‘ contact with animal tissue or
the like from which sound is to be picked up, a
vibratory diaphragm in said casing adjacent the
front thereof and having its periphery connected
therewith, a crystal type microphone element 55
mounted in said case and connected with said
like, compressed into the casing. I have found diaphragm and a leak from the space in front
the felt disc quite satisfactory, however, as it of said diaphragm to the portion of said casing
can be compressed by the back l4 to rigidly re
behind said diaphragm.
tain the microphone element and the parts to
which it is attached within the casing I 0. ‘In
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