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Патент USA US2405179

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Filed Sept. 4, 1941
2 Sheets-Sheet 1-‘
Patented Aug. 6, 1946
Robert Black, Jr., South Orange, and Frank F.
Romanow, Berkeley Heights, N. J ., assignors to
Bell Telephone Laboratories, Incorporated, New
York, N. Y., a corporation of New York
Application September 4, 1941, Serial N 0. 409,444
6 Claims.
This invention relates to electromechanical de
vices, and, more particularly, to a pressure varia
tion detector or microphone for use in a liquid '
medium such as water.
An object of the invention is to improve the
structure and increase the sensitivity of sub
marine signal detectors.
A more speci?c object of the invention is to
enable the use of a small moving coil type micro
phone for submarine signal detection.
(01. 177-385)
the pole-piece and diaphragm assembly of the
microphone unit of the device of Fig, 2;
n ,,
Fig. .5 is an enlarged top plan view of the
assembly of Fig. 4, partly broken away to show
details of diaphragm structure; and
Fig. 6 shows a typical response-frequency char
acteristic for a device actually constructed in
accordance with the invention.
The pressure variations detector device or
1,0 microphone constituting an embodiment of this
An acoustic wave detector or microphone of the
invention is shown in Fig. 2, partly in cross sec
type embodying a small light-weight, high
tion and partly broken away. It comprises an
strength diaphragm having a moving coil at
enclosure, housing or container l0 consisting of
tached to it and arranged in a magnetic air-gap
a front, substantially cup-shaped rigid portion
with which air damping means is associated, has 15 it and a rear, substantially cylindrical rigid por
been found to be a highly e?icient and sensitive
tion l2, joined together at the threaded junc
device for use in a medium such as air. If such
tions M, l5 by a round, shouldered connector
a device is used in a liquid medium such as water,
block or member [3. The front wall [6 of the
it is found that its characteristics are adversely
portion H is provided with a plurality of aper
affected because of the radiation mass of the 20 tures ll whereby pressure variations in the me
water, the radiation mass of air being negligible
dium, namely, water, in which the microphone is
in comparison.
of this invention, this di?iculty is obviated or sub
located have access to the diaphragm structure
of the detector or microphone unit l8. The lat
stantially eliminated by the provision in the
ter is supported rigidly within the portion H
In accordance with one feature
microphone of a second and smaller diaphragm 25 and against the inner surface of the wall l6 by
that is directly exposed to the water and coupled
the threaded clamping ring [9. The unit l8 will
to the inner and larger diaphragm by a very shal
be described hereinafter in greater detail with
low air chamber or space.
reference to Figs. 3, 4 and 5. A water-tight seal
If the microphone is located at any appre~
between the portion H and unit I8 is provided
ciable depth in the water, the hydrostatic pres 30 by the annulus of rubber 69 in a similarly shaped
sure on the moving parts thereof becomes con
recess in the wall It, into which the annular
siderable and may be su?iciently great to deform
ridge 2!] on the outer plate (shown in section in
or permanently damage the diaphragm structure.
Fig. 2) is adapted to be forced. Electrical con
nection between the microphone unit and other
In accordance with another feature of the inven
tion, the microphone is enclosed in a rigid struc 35 electrical equipment or devices to be actuated by
ture, non-resonant in the band of frequencies to
the electrical currents generated by the micro
which the microphone is intended to be respon
phone unit, is made through the cord or cable
2| entering the portion ll through a suitable
water-tight gland or stu?ing box 22, and contain
ing a pair of conductors or wires 23.
The housing portion I 2 is provided with one
sive, the rigid structure enclosing hydrostatic
pressure equalizing means coupled to the rear
or non-exposed areas of the diaphragm struc
ture. The pressure equalization arrangement
may include a bellows member whose volume
varies with the hydrostatic pressure, the water
or more apertures or passages 24 that permit
entrance of the liquid medium into the space or
having access both to the diaphragm structure 45 chamber 25. These apertures, however, are of
such cross section and length as to present a very
and the bellows through the rigid structure. The
high impedance to pressure variations of the fre
water passages to the bellows are of very high
quency range to which the microphone is in
impedance to signal pressure variations in the
tended to be responsive, and such that substan
A more complete understanding of the inven 50 tially no such pressure variations are caused to
exist within the container on the rear side of the
tion will be derived from the detailed description
microphone unit. The connector block contains
that follows, taken in conjunction with the ap
an elongated, restricted passage 26 that consti
pended drawings wherein:
tutes an air or other gas path between the space
Fig. 1 illustrates how the signal detector or
or chamber 21 of portion II and the interior
microphone of this invention may be supported 55 of chamber 28 of a pressure-adjusting member or
under water;
bellows 29, for example, of thin metal, whose
single open end 30 is closed by the connector
microphone in accordance with the invention;
block to which it is securely fastened by being,
- Fig. 3 is a cross-sectional view of the micro
for example, sweated thereon. The connector
phone unit embodied in the device of Fig. 2; 60 block projects into the bellows to reduce the vol
i-Fig. 4 is an enlarged cross-sectional View of
ume of chamber 28 without losing the advantage
Fig. 2 is a partial cross-sectional view of a
silkfin position-under the-air-gap. "Thecoil leads
52 (Fig. 3) are brought out through slots (not
' :shown) in the upper surface of the plate pole
the frequency rangeto which the microphone is
intended to be responsive, and the container as
and are connected with the cord conductors by
the conductive links 53. The assembly 35 is held
a whole is so rigid as to be non-resonant over
at least the same band of frequencies.
The microphone unit is ishowndnrorosssection
in Fig. 3. It comprises a permanent >magnetl3-i,»
that may be of cobalt steel, with a central pole
portion 32, and an outer pole portion 33 contain
ing diametrically located slots 34 therein. These
slots connect the chamber 21 with the space ~64
de?ned by the magnet. The magnet supports a
mole-piece : and ‘diaphragm_.struoture;assemb1y .35,
shown in, enlarged :detail inLFig. 1.4.
{The assembly '35 comprises .an :outer annular
plate pole-piece .136 and ;a {circular center "p018
piece 31 having ;a aCOl'lVBX or dome-shaped :sur
face-38, which maybe ofsoft iron. glt includes a
diaphragm structure ‘consisting .of two dia
phragms 39, 4d, the dimer -diaphragmcomprising
‘.a centrally stiffened .or dome-shaped'portion 4|,
ran annular rim or clamping .portion .42 and .an
intermediate, annular tangentially corrugated
r?exibleportion 43.
The rdiaphragms are, as indicatedby the .draw
i'ingS,‘C0aXia1, and-may .be of :the :same thickness
and material, for example, ‘a ‘light-weight, high
-»strength: material .such as aluminum or can alumi
num ;alloy, for example, ,Duralumin.
15] of acoustic {damping ;material, for vexample,
of the additional corrugations or pleats of the
bellows. "The passage "26 is proportionedtoibe of
very high impedance to pressure variations in
against theitmagnet‘by magnetic attraction, and is
» properly positioned thereon by cooperating pins
~"(not shown) "onthe outer pole of the magnet and
‘recesses '54 :(‘only one shown) in the plate pole.
The ‘microphone ,unit is held against turning in
the portion l I by the pin 55. The restricted space
or chamber 56 between the diaphragms is con
nected with the chamber 21 through asmall leak
15 ageslot 51 in the diaphragmspacer ring or mem
ber ‘48.. 'The :slot 5-‘! :allowsfor :static variation
.in ,pressure but offers a'high ‘impedance to pres
sure variations of the frequency range to'which
.the microphone is intended .to 1be "responsive.
The radiation mass IMR of ‘a diaphragm :is equal
to 8/ 3pa3, where p is the density of the medium
in which the diaphragm ;-is located and id is ‘the
veffective radius of {the diaphragm. In asmedium
such as air,~the radiation mass'issmall-compared
25 to the mass of the diaphragm'and-its associated
moving parts, for example, .a moving coil. ;In a
dense medium such as water, however, this is no
longer the case. If it is desired to employinwa
ter azmicrophone that has been "constructed or
designed for use in air, ‘the ‘increased radiation
.mass must be reduoediin some manner; otherwise,
.The dia
radical changesin the mechanical constants of
the moving parts of the microphone and .of air
;phragm‘s, asshowninfig. 5,=are» circular; the out
»side ,diametersiof'the inner and-outer diaphragms
-may;be in thetratio .ofzapproximately 1.5 to 1; .the
damping means associated with the diaphragm
:radial widths ofvthe ,?exible 'annuli of the :inner 35 become necessary. In accordance with'th'is :in
vention, reduction in the radiation massisaccom
and .outer diaphragms .may .be ,in the ratio 'of 2
.plished with no ‘substantial change in the con
In aimicrophone constructed in accordance
stants of the microphone for "use in airyexcept
@witntheinvention, eachr'dia-phragmwas .001 inch
‘in thickness, diaphragm .4B.had>:an outerdiame- , that :oftheairdamping’means ,(silk) and itsas
.ter of :800 inch and diaphragm ;;outer,di 40 =sooiated mass, which must be increased, ‘by 'ad
-ditionof the second, outer and rsmall diaphragm,
:ameter .of 1.150 inch; the ‘bases .of :the central
whereby a smaller diaphragmarealis:exposedto
.:.sti?ened (portions wereAQOinchtand 37.50inch, re
“the water medium; this smaller diaphragm is'cou
spectively,‘ in i diameter. A ‘ moving :coil 44 of 1 edge
‘:Wound ‘aluminum ribbon iissecured onxits :upper- > -~
.mostiturn to the diaphragm the junctionnf
:the dome-shaped rand :?exible portions thereof.
The outer diaphragm is relativelymore shallow, in
itsicentral portion and smaller in diameter "than
:the inner diaphragm, but also consists .of :a I cen
~p1ed to-thelarger diaphragm through-.anair-spa‘ce
.or chamber :of such size that substantially no nor
;only 1a desired ‘shunting =;e?ect :occurs "with re
spectxto vibration transferred from the outer :to
.the :inner :diaphragm. The effect of the ~.stiif
Y~ .ness added .by ‘thegouter diaphragm to the mov
trally stiffened or vdome-shaped central portion 50 ,ing system of the microphone is to cause a {fall
-ing~;off in'the :response .of ‘the {microphone ,at low
ra45,.1anrannularrimzor mountingportion 46
frequencies compared to theld'evicewith a single
intermediate, annular, tangentially corrugated
flexible portion 41. Theirim'iportion sofftheiinner
diaphragm‘ only. Since the radiation smassivaries
diaphragm is clamped against ‘the plate pole by ’ -as :the -.oube of :the diaphragm radius, ,and the
an annular clamping member "48 of :non-magnetic
added stiffness ascribable to-the iouter‘diaphragm
;material, with vthe 'coil :44 disposed in‘the annular
varies inversely as {the square of :this radius, {the
;air-;gap ‘I0 de?ned by the opposed’verticalsannu
permissible limit to ‘reduction in radiation :mass
i'larsurf aces of the pole-pieces.
.is determined :by thercharacter of frequency re
‘The router ‘di'aphragmiis securelyv fastened at2its 7 :sponse desired rat 'the low frequency :.portion ‘of
“rim :portion .to'the outer :‘surface coffzthe 'member 60 .thjezaudio frequency range.
48. Thecoutersurface of the diaphragm *Mhm'ay
When the microphone is submerged (or im
be :provided with an anodic .?nish.and:be coated
mersed in the liquid medium, there will be (a
‘with a ~water~resistant ivarnish. The :central
,pressure acting on'the diaphragm structure .and
portion .of the router ' diaphragm ‘is of substantially
vcoil {to force them inwardly against .the *magnet
the .same curvature :as the ‘central :area vof lithe 6 .and pole-pieces. Simultaneously, however, :the
wstiffened'portion "of ,the inner .diaphragm, .and is
same head of 1iquid is acting-.onzthe rear~side=of
:so supported ‘by thefmember '48 :as vto rest :onand
.the diaphragm structure; that :-is, ‘the , liquid :?lls
against the inner diaphragm. The contiguous
rthe;por.tion II 2 by entering ‘through ‘the “apertures
vportions of the .diaphragm:may ‘.be fastened to
24 and causes the bellows to adjust -in volume :in
:-gether, for example, with'asuitablercement. ‘The 70 accordance with the liquid ;,pressure at the 3par~
uncle-‘pieces are :maintained in desiredzspaceidre
-ticularwdepth. rilvhis {produces-a gas pressure in
:lation by an annular "plate $49 which is staked
.thesbellows, passage -2.6,ichambers 2,1 and 34, the
sat :a suitable number ‘of ' points 'to both ‘:the = plate
and :the center poles. .The'p1atei49 :IGOILtEIiI'lS'E‘ :space 25 B, :the air-leap ‘and the echambers or :spaces
aamultlplicity'of2apertures'50,;and "holds ~ an annulus 75 :58, v5.9 ‘between ‘the diaphragm V339 :and the pole
pieces that equals or balances the liquid pressure
acting through the apertures in the wall [6. If
the depth of the microphone is changed, the bel
lows will adjust in accordance with the changed
pressure head, and the gas pressure will be altered
to balance the di?erent order of liquid pressure
acting on the exposed portion of the diaphragm
structure. Hence damage to the diaphragm or
directly to the liquid medium, and air damping
means on one side of the larger diaphragm in
cluding means in said container to be acted on
by the liquid medium having ingress through said
second opening to balance the pressure of said
liquid medium acting through said ?rst opening
on said pair of diaphragms, said second opening
being of high impedance to the audio frequency
moving parts of the microphone is avoided, and
the stiifened portions of the diaphragm are 10
3. A microphone for detecting audio frequency
adapted to move substantially piston-like in re
sponse to signal pressure variations in the water.
The pressure on the two sides of the small dia
phragm will be comparable provided the stiffness
disturbances in a liquid medium, comprising a
front hollow portion having an aperture for in
gress of audio frequency disturbances in said
medium, a rear hollow portion, a connector block
of the bellows is less than that of the diaphragm, 15 uniting said portions and containing an elongated
and the volumes of chambers 2'1 and E4 of the
passage of high impedance to audio frequency
microphone small compared with the active
waves, a hollow ?exible member of volume vari
volume of the bellows. The enclosing of the bel
able with liquid pressure on its outer surface, said
lows not only prevents signal pressure Variations
member being secured to said block such that
from acting on the rear side of the diaphragm 20 the variable volume de?ned by said member con
through the bellows but also prevents the bellows’
nects through said elongated passage with the
movements from reacting on the diaphragm
space in the front hollow portion, and diaphragm
through the liquid medium.
means mounted in said front portion with an
When signal pressure variations are produced
outer surface exposed to the liquid medium
in the liquid medium and are incident on the 25 through the front portion aperture and with
diaphragm structure, the two diaphragms re
another portion coupled to the space in the front
spond substantially as a unit thereto, and the
hollow portion, said rear hollow portion contain
resultant movement of the moving coil in the air
ing a passage for ingress of the liquid medium
gap causes currents to be generated in the coil
but presenting a high impedance to audio fre
that correspond to such pressure variations. In 30 quency disturbances.
a speci?c microphone constructed in accordance
4. A transducer for use in a liquid medium,
with this invention, the device evidenced a re
comprising a support, a diaphragm having a cen
spouse-frequency characteristic of the character
tral sti?ened outwardly bowed portion, a rim por
shown in Fig. 6.
tion and an intermediate ?exible portion, a cen
The microphone described hereinabove may be 35 trally apertured clamping member securing the
used as a standard microphone for calibrating
rim portion of the diaphragm against the support,
microphones of similar or diiferent construction
said clamping member having a radially inwardly
intended for use in a liquid medium, and speci?
projecting portion spaced from the diaphragm
cally, under water for detecting marine or sub
flexible portion and surrounding and closely
marine signals or disturbances in the audio fre 40 spaced from the bowed portion of the diaphragm,
quency range. Fig. 1 illustrates a manner in
and a second diaphragm supported on the clamp
which the microphone might be suspended from
ing member and closing the aperture therein, said
a boat in a body of water, the suspension means
second diaphragm having a central sti?ened out
including an eyelet support 60 and spring El.
wardly bowed portion resting on and against a
This disclosure has been made with reference 45 restricted central area of the stiffened portion of
said ?rst diaphragm.
to a speci?c embodiment which at this time is con
sidered to be a preferred one. It will be under
5. In a pressure variation detecting device, a
stood, however. that the invention is not limited
composite diaphragm comprising a pair of cir
thereto, but is of a scope embraced by the ap
cular diaphragms, each of said diaphragms hav
pended claims.
50 ing a centra1 sti?fened outwardly bowed portion
What is claimed is:
and an outer mounting portion, the outwardly
l. A submarine signal device comprising a pair
bowed portion of one diaphragm being of lesser
of diaphragms, one of which is directly exposed
diameter than but of substantially the same cur
to the liquid medium, and in which each dia
vature as the central portion of the other dia
phragm has a central dome-shaped portion and 55 phragm, and a spacer member between the dia
a peripheral portion, the central portion of said
phragm outer portions to maintain the latter
one diaphragm being smaller than that of the
in different planes with the lesser diameter bowed
other diaphragm and resting on and against a cen
portion resting on the central area of the bowed
tral area of the dome-shaped portion of said
portion of said other diaphragm.
other diaphragm, and means to mount the dia 60
6. In a pressure variation detecting device, a
phragm peripheral portions in different planes
composite diaphragm comprising outer and inner
with an air space separating the diaphragm por
diaphragms each of which has a central stiffened
dome-shaped portion and an outer mounting por
tions adjacent the dome-shaped central portions.
2. A microphone for detecting audio frequency
tion, the diaphragms being circular and the di
disturbances in a liquid medium, comprising a 05 ameter and height of the central portion of the
container to be completely surrounded by the
liquid medium having an opening therein for
ingress of audio frequency disturbances and a
outer diaphragm and the radial width of the
mounting portion of the outer diaphragm being
less than those of the corresponding portions of
second opening for the ingress of the liquid me- .
the inner diaphragm, and means to support the
dium into said container, a diaphragm structure 70 mounting portions of the diaphragms in differ
in said container behind said ?rst opening and
ent planes with the central portion of the outer
sealing the container against ingress of liquid
diaphragm axially aligned with and resting on
through said ?rst opening, said diaphragm struc
the central portion of the inner diaphragm.
ture including a pair of diaphragms of di?erent
sizes, the smaller diaphragm only being exposed 75
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