U.S. patent number 3,896,274 [Application Number 05/403,573] was granted by the patent office on 1975-07-22 for electret earphone.
This patent grant is currently assigned to Thermo Electron Corporation. Invention is credited to Freeman W. Fraim, Preston V. Murphy.
United States Patent |
3,896,274 |
Fraim , et al. |
July 22, 1975 |
Electret Earphone
Abstract
A push-pull electrostatic transducer employs imperforate
electret elements affixed to the surface of rigid vented backplates
mounted on opposite sides of a conductive diaphragm.
Inventors: |
Fraim; Freeman W. (Lexington,
MA), Murphy; Preston V. (Barcelona, ES) |
Assignee: |
Thermo Electron Corporation
(Waltham, MA)
|
Family
ID: |
23596264 |
Appl.
No.: |
05/403,573 |
Filed: |
October 4, 1973 |
Current U.S.
Class: |
381/191; 307/400;
381/120; 381/189 |
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 019/00 () |
Field of
Search: |
;179/111E,111R,106
;307/88ET |
References Cited
[Referenced By]
U.S. Patent Documents
Other References
Application of Electrets to Electro-Acoustic Transducers by R. E.
Collins; AWA Technical Review Dec. 1973..
|
Primary Examiner: Claffy; Kathleen H.
Assistant Examiner: D'Amico; Thomas
Attorney, Agent or Firm: Neal; James L.
Claims
Having thus described the invention, what is claimed is:
1. An electroacoustic transducer comprising:
a thin, substantially flexible electrically conductive
diaphragm;
a pair of rigid backplates positioned on opposite sides of said
diaphragm;
a separate imperforate electret bonded to each said backplate along
the surface thereof confronting said diaphragm, each said electret
being imperforate to provide a single continuous surface
uninterrupted by perforations, thereby to maximize the ratio of
surface area of perimeter length;
each said backplate being vented at locations not obstructed by the
electret bonded thereto;
said diaphragm and each of said electrets being electrically
insulated from one another;
means for supporting said diaphragm between said electrets with
sufficient clearance for said diaphragm to vibrate; and
means for electrically connecting individually to said diaphragm
and to each of said electrets.
2. An electroacoustic transducer as defined in claim 1, wherein
said electrets are substantially circular.
3. An electroacoustic transducer as defined in claim 1, wherein
said device is a receiver.
4. An electroacoustic receiver as defined in claim 3, wherein said
device is an earphone.
5. An electroacoustic receiver comprising:
a thin, substantially flexible electrically conductive
diaphragm;
a pair of rigid electrode backplates positioned on opposite sides
of said diaphragm;
a separate imperforate electret bonded to and covering a portion of
each of said electrode backplates along the surface thereof
confronting said diaphragm, each said electret being imperforate to
provide a single continuous surface uninterrupted by perforations,
thereby to maximize the ratio of surface area to perimeter
length;
each of said imperforate electrets being substantially planar and
having a ratio of the longest line along the surface which would
pass through the center of gravity to the shortest line along the
surface which would pass through the center of gravity not
exceeding three;
each said electrode backplate being vented for acoustic airflow at
locations not obstructed by the electret bonded thereto;
said diaphragm and each of said electrets being electrically
insulated from one another;
means for supporting said diaphragm between said electrets with
sufficient clearance for said diaphragm to vibrate; and
means for electrically connecting individually to said diaphragm
and to each of said electrets.
6. An electroacoustic receiver as defined in claim 5, wherein said
electrets are substantially circular.
7. An electroacoustic receiver as defined in claim 5, wherein said
device is an earphone.
8. A hearing aid comprising:
microphone means;
amplifier means;
push-pull driving means;
an electrostatic receiver comprising:
a casing;
acoustic ports;
a thin, substantially flexible electrically conductive
diaphragm;
a pair of rigid backplates positioned on opposite sides of said
diaphragm;
a separate imperforate electret, each said electret being
imperforate to provide a single continuous surface uninterrupted by
perforations, thereby to maximize the ratio of surface area to
perimeter length;
said backplates being vented at locations not obstructed by said
electrets;
said diaphragm and each of said electrets being electrically
insulated from one another;
means for supporting said diaphragm between said electrets with
sufficient clearance for said diaphragm to vibrate; and
means for electrically connecting individually to said diaphragm
and to each of said electrets.
9. An electroacoustic transducer comprising:
an electrically conductive vibratile diaphragm;
a pair of rigid backplates positioned on opposite sides of said
diaphragm, said backplates having plastic bases with an
electrically conductive coating thereon, said coating not exceeding
one micron in thickness;
a separate imperforate electret bonded to each said backplate along
the surface thereof confronting said diaphragm, each electret
comprising an electrically non-conductive member having thermal
expansion characteristics similar to the thermal expansion
characteristics of said base and being imperforate to provide a
single continuous surface uninterrupted by perforations, thereby to
maximize the ratio of surface area to perimeter length, an obverse
surface of said member confronting said diaphragm, an electrically
conductive backing on the reverse surface of said member, said
backing bonding said member to said backplate in surface to surface
contact, each said electret and its associated backplate having
similar thermal expansion characteristics;
each said backplate being vented at locations not obstructed by the
electret bonded thereto;
said diaphragm and each of said electrets being electrically
insulated from one another;
means for supporting said diaphragm between said electrets with
sufficient clearance for said diaphragm to vibrate; and
means for electrically connecting individually to said diaphragm
and to each of said electrets.
10. An electroacoustic transducer as defined in claim 9 wherein
said electrically conductive surface of said electret does not
exceed 0.15 microns in thickness.
11. An electroacoustic transducer comprising:
a closed container;
a thin, substantially flexible electrically conductive diaphragm
mounted at its perimeter inside said container, thereby dividing
said container into two sections;
a pair of rigid backplates mounted inside said container on
opposite sides of said diaphragm with sufficient clearance for said
diaphragm to vibrate, each said backplate together with the walls
of said container forming an acoustic chamber;
a separate imperforate electret bonded to each said backplate along
the central portion of the surface thereof confronting said
diaphragm;
said backplate having holes through it at locations surrounding
said electret for providing acoustic passageways between said
diaphragm and said acoustic chambers; and
an opening in said container for providing an acoustic port
communicating with at least one of said acoustic chambers.
Description
BACKGROUND OF THE INVENTION
The bias necessary to obtain satisfactory linearity in an
electroacoustic transducer can be provided by an electret, either
as a diaphragm or as part of a fixed electrode. When the
displacement of the diaphragm is substantial, linearity of
operation is further enhanced by the push-pull mode of
operation.
One problem encountered with electrets is that they degrade, or
lose their charge with time, under the stresses of manufacturing
processes or during normal operation. Polarization stability of
electrets may be controlled by the choice of manufacturing process,
materials, structure, or a combination thereof.
SUMMARY OF THE INVENTION
The invention pertains to electroacoustic electret transducers and
particularly to a combination of fixed and vibrating elements as
employed in earphones and the like. The general object of this
invention is by structural improvement to extend the useful life of
the electret elements and to simplify the construction of acoustic
devices employing electret elements.
The invention incorporates imperforate electret elements attached
to the surface of rigid vented backplates mounted on opposite sides
of a conductive diaphragm. The invention has the following specific
objects:
1. To maximize electrical discharge pathlengths on the exposed
dielectric surface of the electret elements;
2. To simplify assembly procedure; and
3. To eliminate degradation of the electret elements due to
perforation during the manufacturing process.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a sectional plane view of a preferred embodiment of the
electret earphone of this invention.
FIGS. 2a) through 2c) show a plane view of a number of preferred
embodiments of electret elements on suitable backplates.
FIG. 3 is a schematic of the earphone of FIG. 1 embodied in a
hearing aid.
DETAILED DESCRIPTION OF THE DRAWINGS
The invention will be described in detail referring to FIG. 1. An
earphone 10 includes a casing 11 which forms acoustic chambers 12.
An acoustic port 13 is provided for sound output. A sound absorbing
material 15 fills a portion of the acoustic chambers 12 to provide
acoustic damping and protection against dust intrusion. Within the
casing 11, a conductive diaphragm 14 is tautly suspended by
elements 16. On each side of the diaphragm 14, an electrically
conductive backplate 18 is mounted and isolated from the casing 11
by elements 20. A film electret element 22 is affixed to the inner
surface 24 of backplate 18. Acoustic passages 25 in the backplates
18 are provided at locations not obstructed by electret elements
22. A capillary 23 equalizes pressure between acoustic chambers 12.
A surface 26 of the electret 22 faces the diaphragm 14. The
diaphragm 14 and electrets 22 are electrically insulated from one
another by elements 16 and 20. Conductors 28 connect the backplates
18 and the diaphragm 14 to a representative push-pull network 30,
shown schematically. The network 30 is connected to a signal source
32.
Operation of the apparatus of FIG. 1 will now be described. A
signal from the source 32 is received through the push-pull network
30 via conductors 28 connected to the diaphragm 14 and the
electrodes 18. At the electrets 22, the signal is biased by the
persisting field in the electrets 22. The signal causes a
time-varying electric field to develop between the electrets 22 and
the diaphragm 14 which induces the diaphragm 14 to vibrate. The
vibration induces sound waves in the acoustic cavity 12. The
resultant sound passes through the acoustic port 13.
In a preferred embodiment, the dimensions of the earphone 10 are
approximately 10 mm in length by 10 mm in width by 5 mm in depth.
The earphone 10 as shown has a metal casing 11. The casing 11 may
be plastic if no electrostatic shielding is required or size is not
a constraint.
The diaphragm 14 is the sole vibratile element. It is typically 6
microns in thickness and is a plastic film 17 with metallized
surfaces 31. Other materials may be used so long as the diaphragm
14 is electrically conductive and sufficiently flexible.
The electret elements 22 are dielectric films typically 10 microns
to 25 microns in thickness. A thin electrically conductive backing
27 on the dielectric base 29 of the electret element 22 assures
good electrical contact between the electret element 22 and the
backplate 18. The electrets 22 are bonded to the backplates 18 by
an appropriate adhesive. The spacing between the electrets 22 and
the diaphragm 14 may vary depending on the desired acoustic and
electrical characteristics. The electret 22 is characterized by a
persistent electrostatic charge. The dominant component of the
charge, called the homocharge, resides on or near surface of the
electret 22. Since the planar electret 22 has no perforations, the
ratio of surface area to perimeter length is relatively high, and
the electrical discharge path along the surface to any edge is
maximized. Several preferred shapes of electrets 22 are shown in
FIGS. 2a through 2c. The acoustic air passages 25 in the backplates
18 may be of various shapes and sizes as shown in FIGS. 2 a through
2 c. More than one electret 22 may be bonded to each backplate 18
as shown in FIG. 2 c in order to permit the central location of the
air vents 25 and to shape the vibration characteristics of the
diaphragm 14.
An electrically conductive backplate having thermal expansion
characteristics closely matching those of the electret element
affixed thereto is desirable in order to inhibit deterioration of
the electret element caused by thermally induced stress. This is
accomplished in the backplates 18 by providing a plastic base 19,
which has thermal expansion characteristics matching those of the
electrets 22, with an electrically conductive coating 21 less than
one micron in thickness. A gold coating of 0.06 microns is
suitable. Such a coated backplate has essentially the thermal
expansion characteristics of the uncoated plastic. Likewise an
electrically conductive backing 27 less than 0.15 microns thick on
the electret elements 22 does not have an substantial adverse
effect on the thermal expansion characteristics of the electret
elements 22. If the backplates 18 are not electrically conductive
or do not have electrically conductive surfaces 21, electrical
connections 28 must be made directly to the electret elements
22.
FIG. 3 illustrates an embodiment of the invention in a hearing aid
34. A microphone 36 is connected to an appropriate amplifying means
38. The output is connected to the earphone 10 embodying the
invention.
The invention herein described has the following advantages:
1. the electrical discharge pathlengths along the surface of the
electrets are maximized so that deterioration of the electret is
impeded;
2. the assembly procedure has been simplified by avoiding the
necessity of critical alignment of elements with holes or the
formation of holes in the electret during or subsequent to
assembly, and correspondingly
3. deterioration of electrets induced by the manufacturing process
is eliminated.
While the invention has been described with respect to the details
of a specific embodiment thereof, many changes and variations will
be apparent to those skilled in the art, and such can obviously be
made without departing from the scope of the invention.
* * * * *