U.S. patent number 3,816,671 [Application Number 05/241,580] was granted by the patent office on 1974-06-11 for electret transducer cartridge and case.
This patent grant is currently assigned to Thermo Electron Corporation. Invention is credited to Freeman W. Fraim, Preston V. Murphy.
United States Patent |
3,816,671 |
Fraim , et al. |
June 11, 1974 |
ELECTRET TRANSDUCER CARTRIDGE AND CASE
Abstract
An electret microphone is made of a cartridge subassembly
including an electret diaphragm fastened to an apertured backplate
having diaphragm support means and air vent means for balancing
static pressure on both sides of the diaphragm as unitary parts of
the plate. This subassembly is held in a housing between a
diaphragm contact ring that grounds the metal diaphragm component
to the housing and a resilient compression ring that bears on the
back plate and the air vent is totally within the housing. The
front of the housing has one of several enclosures over the
diaphragm for frequency response varying from the voice range to
the high fidelity range. A back closure plate, bounding the
compression chamber, carries a solid-state preamplifier and a
ring-shaped resilient conductor that makes rolling contact with the
back-plate electrode of the cartridge.
Inventors: |
Fraim; Freeman W. (Lexington,
MA), Murphy; Preston V. (Weston, MA) |
Assignee: |
Thermo Electron Corporation
(Waltham, MA)
|
Family
ID: |
22911276 |
Appl.
No.: |
05/241,580 |
Filed: |
April 6, 1972 |
Current U.S.
Class: |
381/114; 381/173;
307/400 |
Current CPC
Class: |
H04R
19/01 (20130101); H04R 25/604 (20130101) |
Current International
Class: |
H04R
19/01 (20060101); H04R 19/00 (20060101); H04R
25/00 (20060101); H04r 019/04 () |
Field of
Search: |
;179/111R,111E,106,179
;307/88ET |
References Cited
[Referenced By]
U.S. Patent Documents
Foreign Patent Documents
Primary Examiner: Claffy; Kathleen H.
Assistant Examiner: Kundert; Thomas L.
Attorney, Agent or Firm: Neal; James L.
Claims
What is claimed is:
1. An electroacoustic transducer comprising:
a. a cartridge, said cartridge comprising a pair of plates mounted
in confronting relation, a first of said plates comprising an
electret having a dielectric surface confronting a second of said
plates and an opposed metallized surface, and being imperforate and
flexible to serve as a diaphragm, said second plate comprising a
relatively rigid dielectric body pierced for acoustic airflow to
serve as a backplate and having on the surface thereof confronting
said dielectric surface a peripheral ridge extending to the
peripheral region of said dielectric surface and a supporting
element extending from said confronting surface within and spaced
from said peripheral ridge to said dielectric surface for making
contact with and supporting said first plate in spaced relation
from said second plate, a metallic coat on said confronting surface
of said body within said ridge and conductor means extending
through said body for making electrical contact to said coat from
the opposed side of said body, the surface of said ridge
confronting said peripheral region of said dielectric surface being
free of said coat, and means effecting a dielectric-to-dielectric
bond between said surface of said ridge and said peripheral
region;
b. a tubular housing having at one end electrically conductive
means at least partially closing said one end, said cartridge being
disposed in said housing with said metallized surface of said first
plate facing to said one end;
c. an electrically conductive belt-like contact member in
electrical contacting relation to said metallized surface of said
first plate in a peripheral region thereof opposed to said
peripheral ridge of said second plate and bearing against said
closing means;
d. a rigid third plate disposed within said housing at the other
end to serve as a closure for said housing;
e. an electronic signal transducing circuit formed on the surface
of said third plate within said tubular housing, said circuit
having a first terminal on said surface, and a second terminal on
the opposed surface of said third plate, and resilient electrical
conductor means comprising resilient dielectric material loaded
with electrically conductive material held under compression
between said second and third plates making connection between said
first terminal and said electrical contact to said coat on said
second plate;
f. resilient belt-like means in said housing under compression
between the respective peripheries of said third plate and said
opposed side of said body of said second plate; and
g. means holding said third plate fast in said housing, the space
between said first and third plates providing a compression chamber
for said diaphragm.
2. An electroacoustic transducer according to claim 1 wherein said
electrically conductive belt-like member comprises a a dielectric
body coated with an electrical conductor.
3. An electroacoustic transducer comprising:
a. a cartridge, said cartridge comprising a pair of plates mounted
in confronting relation, a first of said plates comprising an
electret having a dielectric surface confronting a second of said
plates and an opposed metallized surface, and being imperforate and
flexible to serve as a diaphragm, said second plate comprising a
relatively rigid dielectric body pierced for acoustic airflow to
serve as a backplate and having on the surface thereof confronting
said dielectric surface a peripheral ridge extending to the
peripheral region of said dielectric surface and a supporting
element extending from said confronting surface within and spaced
from said peripheral ridge to said dielectric surface for making
contact with and supporting said first plate in spaced relation
from said second plate, a metallic coat on said confronting surface
of said body within said ridge and conductor means extending
through said body for making electrical contact to said coat from
the opposed side of said body, the surface of said ridge
confronting said peripheral region of said dielectric surface being
free of said coat, and means effecting a dielectric-to-dielectric
bond between said surface of said ridge and said peripheral
region;
b. a tubular housing having at one end electrically-conductive
means at least partially closing said one end, said cartridge being
disposed in said housing with said metallized surface of said first
plate facing to said one end;
c. an electrically conductive belt-like contact member in
electrical contacting relation to said metallized surface of said
first plate in a peripheral region thereof opposed to said
peripheral ridge of said second plate and bearing against said
closing means;
d. a rigid third plate disposed within said housing at the other
end to serve as a closure for said housing;
e. an electronic signal transducing circuit formed on the surface
of said third plate within said tubular housing, said circuit
having a first terminal on said surface, and a second terminal on
the opposed surface of said third plate, and resilient electrical
conductor means held under compression between said second and
third plates making connection between said first terminal and said
electrical contact to said coat on said second plate, said
resilient conductor comprising an electrically conductive
elastomeric ring affixed at its periphery to said first terminal
with its axis of revolution substantially parallel to said surface
of said third plate within said tubular housing, whereby when
placed under compression said ring is distorted in a direction
transverse to said axis;
f. resilient belt-like means in said housing under compression
between the respective peripheries of said third plate and said
opposed side of said body of said second plate; and
g. means holding said third plate fast in said housing, the space
between said first and third plates providing a compression chamber
for said diaphragm.
4. An electroacoustic transducer according to claim 3, wherein said
electrically conductive belt-like member comprises a dielectric
body coated with an electrical conductor.
5. An electroacoustic transducer comprising:
a. a cartridge, said cartridge comprising a pair of plates mounted
in confronting relation, a first of said plates comprising an
electret having a dielectric surface confronting a second of said
plates and an opposed metallized surface, and being imperforate and
flexible to serve as a diaphragm, said second plate comprising a
relatively rigid dielectric body pierced for acoustic airflow to
serve as a backplate and having on the surface thereof confronting
said dielectric surface a peripheral ridge extending to the
peripheral region of said dielectric surface and along the opposed
side thereof away from said diaphragm at least one groove extending
to the periphery of said rigid body providing an air leak for
permitting air communication from one side of said diaphragm to the
other, a metallic coat on said confronting surface of said body
within said ridge and conductor means extending through said body
for making electrical contact to said coat from the opposed side of
said body, the surface of said ridge confronting said peripheral
region of said dielectric surface being free of said coat, means
effecting a dielectric-to-dielectric bond between said surface of
said ridge and said peripheral region, and means to make electrical
contact to said metallized surface of said first place;
b. a tubular housing having at one end closure means at least
partially closing said one end;
c. means supporting said cartridge in said housing with said
diaphragm facing to said closure means and spaced therefrom;
and
d. air passage means between the inner-side walls of said housing
and the confronting outer peripheral walls of said cartridge
communicating with said air leak for permitting air passage from
said opposed side of said cartridge to the region between said
diaphragm and said closure means.
6. An electroacoustic transducer according to claim 5 in which a
transverse dimension of said cartridge is smaller than the
corresponding inner transverse dimension of said housing, to
provide said air passage means.
7. An electroacoustic transducer according to claim 6 including
spacer means between said diaphragm and said closure means for
locating said cartridge relative to said closure means, a third
rigid plate disposed within said housing at the other end to serve
as a back closure plate for said housing, resilient belt-like means
in said housing under compression between the respective
peripheries of said closure plate and said opposed side of said
body of said second plate, said resilient belt-like means forming a
seal for establishing a sole fluid communication passage from one
side of said diaphragm to the other through said groove, and means
holding said closure plate fast in said housing, the space between
said first and third plates providing a compression chamber for
said diaphragm.
8. An electroacoustic transducer according to claim 5 in which said
housing closure means at said one end has an aperture which extends
over the major portion of said metallized diaphragm surface.
9. An electroacoustic transducer according to claim 5 in which said
housing closure means at said one end overlies substantially all of
said metallized diaphragm surface, said closure means having an
aperture of which the area is a minor fraction of the area of said
metallized surface, thereby defining a substantially fully enclosed
volume of air between said housing closure means and said
diaphragm.
10. An electroacoustic transducer according to claim 5 having a
tubular conduit extending laterally from said housing into the
region between said housing closure means at said one end and said
metallized diaphragm surface, said closure means fully closing said
one end of said housing and covering entirely said metallized
diaphragm surface, thereby defining a substantially fully enclosed
volume of air between said housing closure means and said
diaphragm.
11. An electroacoustic transducer comprising an electrically
conductive housing having a front end, a back end, and side walls,
a substantially imperforate and flexible electret diaphragm
supported in the front end of said housing, electrostatic shield
means overlying said diaphragm in said front end, closure means for
said back end, said closure means including electrostatic shield
means along one surface thereof electrically connected to said
housing, power terminal means overlying said shield means, an
insulating film covering a portion of said shield means separating
said power terminals and said shield means, and ground terminal
means engaging said shield means.
Description
BACKGROUND OF THE INVENTION
Electrostatic transducers, such as microphones and speakers, have
reached the state where, by using electrets, they can dispense with
a polarizing voltage supply. Examples are described in U.S. Pat.
No. 3,612,778, entitled "Electret Transducer and Method of Making,"
which is owned by the assignees of this application. With this
development comes the possibility to make very small, rugged and
lightweight transducers exhibiting the broad-band response that is
regarded as "high fidelity," as well as narrower response ranges
suitable for voice communications. As the attempt is made to
fabricate and assemble smaller sizes, however, mechanical and
manipulative problems are confronted, and it becomes increasingly
difficult to satisfy all the desired specifications of acoustic
properties, mechanical and thermal properties, ease of assembly and
low cost.
BRIEF SUMMARY OF THE INVENTION
In this invention an improved electret cartridge is presented which
in a single subassembly makes provision for desired acoustic,
mechanical, thermal and assembly properties. The cartridge itself
is straightforward to make. It fits into a housing which can have
any one of several designs depending on the intended use and
desired response bandwidth. Conveniently, the housing has
rectangular outer dimensions, while providing a circular chamber
for electro-mechanical components. When the cartridge is fitted
into a housing, air leaks from the front to the back of its
diaphragm are automatically provided, and the cartridge is held
under rim-compression in an operative position, with electrical
connections to it reliably made. The housing closure carries on one
surface within the housing a solid-state amplifier circuit which
is, upon assembly, coupled to the cartridge. The invention is
illustrated in a microphone having outer dimensions slightly larger
than 1/2 inch square by 1/8 inch thick.
BRIEF DESCRIPTION OF THE DRAWINGS
Fig. 1 is an enlarged exploded longitudinal sectional view of
un-cased components of a transducer according to the invention;
Fig. 2 shows an exploded oblique view of a transducer according to
the invention and an electrical schematic circuit as embodied in
certain of the parts;
Fig. 3 shows at A, B and C, respectively, a longitudinal section, a
bottom and a side view of an assembled transducer;
Fig. 4 shows a modification of the FIG. 3 transducer, in the
acoustic portion to the left of dashed line X--X; and
Fig. 5 shows another modification of the acoustic portion of the
FIG. 3 transducer.
DETAILED DESCRIPTION OF THE DRAWINGS
In FIG. 1, the components of an electret cartridge 10 are shown
separated from each other to facilitate a clear description of
them. The electret 11 is made of a dielectric sheet 12 coated with
an electrical conductor 13, here gold, put on for example by
vapor-deposition in vacuum to a thickness which typically is
between about 01. and 0.01 micron. Electrets suitable for use in
the invention, and methods of making them, are described in the
aforesaid U.S. Pat. No. 3,612,778. A back plate 15 is made of a
single dielectric body 16, which in the present example is a
plastics material formed by injection molding, bearing an
electrically conductive coat 17, again of gold, for example. The
back plate is perforated with several apertures 18 to provide air
passages through it; these apertures are preferably formed in the
process of molding the dielectric body. A ridge or land 20 extends
toward the electret 11 from the peripheral part of the inner
surface 21 of the body 16 confronting the surface 19 of the
dielectric sheet 12 of the electret. In the case where the
cartridge 10 is circular, as in the present example, this ridge 20
is annular in form. If not annular, it may be generally regarded as
having a belt-like form. A central projection or post 22 extends
from the inner back-plate surface 21 similarly to the ridge 20. The
electrically-conductive coat 17 covers the inner surfaces of the
apertures 18, the face of the inner back-plate surface 21 not
including the projected faces of the ridge 20, and the back or
outer surface 24 of the dielectric body 16 nearly but not entirely
to the side walls 25.
In practicing the invention, the electret 11 is brought in contact
with the back plate 15 and while held under tension to keep it taut
the two parts are fastened together to form a
dielectric-to-dielectric bond between the ridge 20 and the
peripheral portion of the electret dielectric surface 19. Cements
can be used, in which case suitable cements are epoxies and
acrylates. The dielectric body 16 may be made of a plastics
material known as "NORYL," a trademark of General Electric Co. for
injection molding granules believed to be composed of polyphenylene
oxide and polystyrene. Preferably, this body is molded to a
tolerance of 1 mil-inch. Suitable materials for the electret
dielectric 12 are mentioned in the above referenced U.S. Pat. No.
3,612,778. A plastic-to-plastic, or dielectric-to-dielectric, bond
is formed. The center post 22 extends flush into or slightly beyond
the plane of the outer surface of the ridge 20, but need not be
cemented or otherwise affixed to the electret diaphragm. The
electret 12 is thus bonded at its periphery to the outer land or
ridge 20 and supported on the center post 22, and is free to move
in the recesses annular region of the inner surface 21 of the back
plate 15.
A metallized plastic ring 30 confronts the electret conductor 13 in
a peripheral region somewhat narrower than the surface of the ridge
20 bonded to the dielectric side 19 of the electret. The body 31 of
this ring is made of a plastics material chosen to have thermal
properties compatible with those of the back-plate body 16, and it
is coated with an electrical conductor 32, e.g: gold, for coupling
the electret conductor 13 to a housing (to be described). A gold
layer having suitable thickness for use as the electret conductor
would be too thin to allow the making of a gold-to-gold bond to the
electret. In practice the ring 30 is compressed against the
cartridge 10, its compressive force being exerted against the
back-plate ridge 20, so that the electret, in addition to being
affixed to the back plate, is held in place by compressive force.
The aperture 34 in the ring 30 is an acoustic port for the
electret.
Two spacers 36 and 37 are provided for use in assembling the
electret 10 and conductor ring 30 into a case. The first spacer 36
is a ring which may be made of TEFLON, trademark of E. I. DuPont de
Nemours & So., for a plastic consisting of a
tetrafluoroethylene polymer. The second spacer 37 is a ring which
may be made of resilient electrically non-conductive rubber, the
axial dimension of which is so chosen that in an assembled
transducer this spacer will be under compression in the direction
of its axis of revolution. For reasons that will presently appear
(see FIG. 3) the first spacer ring 36 has the same outer diameter
as the electret cartridge 10 and its conductor ring 30, but the
second spacer ring 37 has a larger outer diameter. For the same
reasons, the back-surface 24 of the back-plate 15 is fitted with
one or more radial slots (see FIG. 2) extending to the side walls
25 from the aperture 18. The larger ring 37 makes an air-tight seal
against the inner wall of the case 50 to assure that venting occurs
substantially only through these slots and forward to the front
surface of the diaphragm 11, and not along other paths. This is
done so that the frequency response will be predictable as a
function of the slot size.
A closure plate 40, made preferably of a rigid ceramic material
suitable for carrying printed electrical circuitry, is held against
the second spacer 37 in an assembled transducer, with force
sufficient to compress the second spacer. As will presently appear,
in an application of the invention to a microphone, a preamplifier
circuit (FIG. 2) is mounted on the inner surface 41 of the closure
plate, and a ring 42 of electrically conductive rubber (e.g: a
rubber which is loaded with silver) is affixed to the input
terminal of that circuit. The ring is oriented with its axis of
revolution parallel to the surface 41 and it is affixed at its
periphery to the closure plate. Its diameter is such that when the
closure plate is in place in an assembled transducer the ring 42 is
brought in contact with the back-plate conductor 17 at the rear
surface 24, and is distorted in a direction transverse to its axis
of revolution (that is, radially) by compressive force between the
back plate and the closure plate. This provides a rolling contact
with the back-plate conductor 17, which is superior to a wiping or
a rigid contact. The closure plate will have several electrical
contacts extending from its inner surface 41 to its outer surface
43, for connection to power supply and utilization conductors (not
shown). One such contact 44 is shown, and this may be assumed to be
a signal output terminal for the preamplifier. When the parts shown
in FIG. 1 are assembled together as described above, the holes 18
in the back plate serve as acoustic elements to connect the air
volume behind the back plate, bounded by the closure plate 40 and
the spacers 36 and 37, to the air volume within the electret
cartridge 10. The size of these holes, their position and number,
are chosen to obtain a specific frequency response. The
above-mentioned slot (or slots) in the rear surface 24 of the back
plate acts (or act) as an air leak connecting the air volume behind
the back plate with the ambient at the front conductor 13 surface
of the electret. The effective size of this air passage is chosen
to obtain a specific frequency response.
In FIG. 2, the components of a complete transducer, including a
case 50 are shown in exploded view, together with an electrical
circuit diagram showing the electrical function of the electret and
preamplifier. The case is rectangular and made of metal, having a
circular aperture 51 for reception of the conductor ring 30, the
electret cartridge 10, and the spacers 36 and 37, in that order.
Behind the circular aperture is a flanged square opening 52 for
reception of the closure plate 40 which is square in shape.
Clearly, the parts in the circular aperture 51 are dimensioned so
that when the closure plate 40 is absent the rubber ring 37 extends
out of the circular aperture into the square opening 52. The square
opening has a back wall 53 and opposed side walls 54, 55.
Upstanding on each side wall is a foldable flap 56, 57,
respectively, for clamping the closure plate in place as appears in
more detail in FIG. 3. The closure plate has three terminal
conductors 44, 45 and 46 along one edge, and one side of the square
opening 52 is left open, that is, without a side wall, to give
ready access to these conductors.
The circuit schematic in FIG. 2 illustrates that in the electret
cartridge 10 the dielectric diaphragm member 12 has a permanent
electric charge built into it. Electrical contact is made from the
diaphragm conductor 13 to ground terminal 45, and from the back
plate 15 to the input 61 of a preamplifier 60, having a power
supply terminal 46 and the signal output terminal 44. The
preamplifier 60 employs a well-known source-follower circuit. The
electrical circuit details of the preamplifier circuit are not part
of the present invention, and the illustrated circuit is exemplary
only. It is realized in the form of an integrated circuit formed
and mounted on the inner surface 41 of the closure plate 40.
Certain physical or mechanical features of making connections, as
to the case and a ground terminal, and connecting to the input of a
suitable preamplifier circuit are parts of the present invention,
as has appeared in part above. Thus, the conductor 62 connecting
the electret cartridge 10 to the input terminal 61 is in practice
according to the invention realized with the conductive rubber ring
contact member 42, which is permanently bonded to the closure plate
40 and connected to the preamplifier input terminal 61, and which
makes a rolling contact with the back-plate conductor 17. The input
circuit resistor 65, to be suitable for use with an electret signal
source, will be unusually high, for example 2,000 megohms. This is
conveniently made by an available thick-film screening technique
directly on the inner surface 41 of the closure plate. The contact
ring 42 is affixed to one end of this resistor. This combination of
a high-resistance input resistor for the preamplifier, with a
resilient conductor affixed to the input terminal and making
rolling contact under pressure to the cartridge is a novel
arrangement that provides low-noise and mechanical stability
properties to a microphone made according to the invention, as will
be appreciated more fully from the description of FIG. 3 that
follows.
The outside surface 43 of the closure plate is metallized and has a
coat (not shown) that is electrically conductive, and this coat
contacts the casing 50 via the folded flaps 56, 57 (see FIG. 3) to
form with the casing an electrostatic shield for the microphone. An
insulating film (48, in FIG. 3B) separates the signal and power
terminals 44 and 46, respectively, from the metallized coating. A
portion of the ground contact 45 engages the conductive metallized
coating so that contact between the diaphragm conductor 13 and the
ground terminal 45 is thus established through the metallized
coating, flaps 56, 57, the housing and the front conductor ring 30.
A second alternative conductive path between the diaphragm
conductor 13 and the ground contact 45 is made by an electrically
conductive adhesive (not shown) located between the housing in the
square aperture 52 and the part of ground contact 45 which folds
under the closure plate to overlie the inner surface 41 (similar to
contact 44 shown in FIG. 1).
An assembled transducer, here a microphone embodying the parts and
circuit shown in FIG. 2, is shown in FIG. 3. One foldable flap 57
is shown in FIG. 3A folded over the closure plate 40; both flaps 56
and 57 are shown in FIGS. 3B and 3C. FIG. 3A is taken on line A--A
in FIG. 3B. The housing 50 has a front wall 58 terminating the
circular aperture 51, against which the contact ring 30 bears when
the rubber spacer 37 is under compression from the closure plate
40. The front wall is fitted with one or more air-leak slots 71
running from the ambient region in front of the electret conductor
13 to the side wall of the circular aperture 51. The outer
diameters of the contact ring 30, cartridge 10 and first spacer
ring 36 are all slightly smaller than the inner diameter of the
circular aperture 51, within the permissible range of dimensional
tolerances, thereby providing an annular air-leak passage 70 from
the front air-leak slots to the radial slot or slots 72 in the rear
surface 24 of the back-plate 15. The locations of two such slots 72
are shown in dotted line in FIGS. 3A and 3B. The continuous
air-leak passage thus formed prevents the build-up of differential
pressure between the air inside the transducer housing and the
ambient.
It will also be seen in FIG. 3A that the conductive contact ring 42
is compressed radially, making a broad rolling-type contact with
the back-plate conductor 17 when the transducer is assembled. No
friction or scratching contact is involved. The fixed contact
between the contact ring 42 and the preamplifier input terminal 61
is not strained, either during assembly, or later during use of the
transducer. This combination of features reduces the probability of
noise arising from a contact, especially in a microphone
incorporating an integrated circuit preamplifier as shown for
example in FIG. 2. This design, combined with an electret condenser
cartridge 10, installed in a suitably rigid housing 50, provides a
virtually shock-proof microphone capable of withstanding repeatedly
a 6-foot drop on a tiled or hardwood floor without damage or
impairment of response.
A typical electret-condenser microphone as shown in FIG. 3 is about
1/4 inch square (FIG. 3B) and about half that much thick (FIG. 3A,
FIG. 3C). The front cover 58 of the housing 50 can take one of
several forms, depending on desired frequency-band response and on
the intended use. FIG. 3 shows a configuration suitable for use in
a hearing aid. The front cover 58 is provided with a small aperture
76 which has a cross-sectional area that is a minor fraction of the
electret diaphragm area, connecting the air volume 75 between the
front cover and the electret cartridge 10 with the environment.
Frequency response of a microphone using this design will be from a
low of about 50 Hz to a high of about 8,000 Hz.
A modification of FIG. 3 to provide a microphone suitable for music
use is shown in FIG. 4 where the front cover 58 has a large
aperture 77 nearly coextensive with the electret diaphragm area, so
that the electret diaphragm is essentially in direct contact with
the environment. In this case it is appropriate to provide a
protective screen 78 in the front cover. This screen will desirably
be electrically conductive, and grounded to the housing 50, so that
it can function as an electrostatic shield. Frequency response of
this design is between about 50 Hz and 16,000 Hz.
In FIG. 5 the front cover 58 is not perforated or opened at all. A
tubular extension 79 at the side of the housing 50 near the front
communicates through the housing with the confined air volume 75
confronting the electret diaphragm. This design is useful for
telephone headsets in which a speaking tube (not shown) is attached
to the tubular extension 79 and extends from a region over the
user's ear to a region near the user's mouth, thus providing an
extremely small and light-weight microphone assembly. Frequency
response of a microphone using this design is from 50-6,000 Hz.
Microphones made as shown in FIG. 3 or FIG. 5 enclose a quantity of
air in front of the diaphragm which can change only slowly with
changes in outside atmospheric or ambient pressure, access to the
front enclosure being severely restricted by the small opening 76
or tube 79. If the back of the cartridge 10 were vented directly to
the ambient permitting rapid changes of back pressure with changes
in ambient pressure, then a sudden drop in the ambient pressure (as
in an explosively depressurized aircraft, for example) would create
a sharply-rising differential pressure on the front of the
diaphragm which could damage or destroy it. The totally-enclosed
air-leak 71-70-72, with no other venting to the atmosphere,
prevents the build-up of such a differential pressure, and
maintains essentially the same static pressure on both sides of the
diaphragm. Thus, for embodiments of the invention like those shown
in FIGS. 3 and 5, a simple bleed to the atmosphere is just not good
enough.
* * * * *