US 3740496 A
A diaphragm assembly for an electret transducer wherein the diaphragm comprises a movable plate, and a compliant surround on the periphery of the plate. The diaphragm rests on supporting members of a backplate and is positioned to obtain a suitable stiffness of the vibrating portions without the use of a taut or tensioned diaphragm.
Description (OCR text may contain errors)
United States Patent Carlson et al.
[ June 19, 1973 DIAPHRAGM ASSEMBLY FOR ELECTRET TRANSDUCER Inventors: Elmer Victor Carlson, Prospect Heights; Mead Clifford Killion, Elk
Grove Village, both of ill. 60007 Industrial Research Products, Inc., Elk Grove Village, Ill.
Filed: Nov. 8, 1971 Appl. No.: 196,592
US. Cl 179/111 E, 179/181 Int. Cl .f. H04r 19/00 Field of Search 179/111, 181, 115.5 ES,
References Cited UNITED STATES PATENTS 5/1972 Madsen 179/111 3,222,463 12/1965 Giirike 179/181 2,567,889 9/1951 Moreland 179/111 FOREIGN PATENTS OR APPLICATIONS 1,212,157 3/1966 Germany 179/181 Primary Examiner-Kathleen H. Claffy Assistant Examiner-A1an Faber Att0rneyWilfred S. Stone I 57] ABSTRACT 7 Claims, 6 Drawing Figures IIL/ DIAPHRAGM ASSEMBLY FOR ELECTRET TRANSDUCER BACKGROUND OF INVENTION In the prior art, the diaphragm and backplate for electret transducers are maintained at proper spacing by affixing the edges of the diaphragm to separated supports and keeping the central portion of the diaphragm from resting on the backplate. A practical difficulty arises in the prior art because the relative dimensions of the diaphragm change as a function of temperature, humidity and aging and, accordingly, the tension on the diaphragm is a function of the foregoing parameters. Since the tension on the diaphragm is a factor in controlling the stiffness of the portions at the diaphragm between the separating supports it becomes an important consideration in the acoustical performance of the transducer.
In the present invention, the diaphragm comprises a relatively rigid plate with supporting members positioned between the diaphragm and the backplate to obtain a suitable stiffness of the vibrating portions without the use of a taut or tensioned diaphragm. A flexible and compliant surround which is formed on the periphery of the diaphragm plate assures that there is no appreciable tension on the diaphragm plate. The surround provides a compliant region to minimize the effect of any dimensional changes of the diaphragm relative to those of the backplate due to changes in temperature, humidity and aging of the materials themselves.
This compliant surround also functions as a selfadjusting member to permit the diaphragm and backplate to be located in proper position without the need of extremely high precision in the dimensions of the backplate, the electret film, and of the diaphragm.
The surround may be metalized such that it can provide an electrical terminal or electrode to be connected to the associated electronic circuit.
Accordingly, it is a principal object of the present invention to provide an improved diaphragm for an electret transducer.
It is another object of the present invention to provide an improved mounting for a diaphragm for an electret transducer comprising a flexible surround en'- abling the diaphragm to function as a plate.
It is another object of the present invention to provide a diaphragm for an electret transducer which provides stable, response characteristics independent of change in temperature, humidity and aging.
The foregoing and other objects, features and advantages of the invention will be apparent from the following more particular description of the preferred embodiments of the invention, as illustrated in the accompanying drawings wherein:
FIG. 1 is a view partiallyin cross-section showing a transducer in accordance with the invention;
FIG. 2 is a plan view of the diaphragm and surround of the invention;
FIG. 3 is relatively enlarged view in cross-section showing the inventive diaphragm construction;
FIG. 4 shows a second embodiment of the inventive diaphragm construction;
FIG. 5 shows a third embodiment of the inventive diaphragm construction; and,
FIG. 6 shows another embodiment of the inventive diaphragm construction. 7
DESCRIPTION OF THE INVENTION Referring to FIG. 1, the electret transducer 11 of the invention comprises a casing 13, which in one embodiment is of an essentially rectangular shape. Either or both of the casing portions 16 and 18 have suitable apertures, not shown, which provide acoustical communication externally ol the casing 13.
A transducer or motor assembly 12 is mounted on a bulkhead 15 positioned in casing 13. Bulkhead 15 may be made of any suitable material and has one or more acoustical passages 30 formed therethrough communicating with a first interior sound cavity 25 formed between bulkhead 15 and the casing portion 18. A second sound cavity 27 is formed between the surface of a vibratable diaphragm assembly 19 and easing portion 16.
FIG. 3 shows a relatively enlarged view of the diaphragm assembly 19 with the thickness of the components being exaggerated to more clearly explain the construction. Diaphragm assembly 19 includes a diaphragm plate 20 (see FIG. 2) which may be made of polyethylene terephthlate commonly available under the trademark Mylar, or of any similar material. Plate 20 is coated on its lower (as oriented in FIGS. 1 and 3) surface with a metalizing layer 24 of conductive material, which, for example, may be evaporated unto the surface. A surround 29, to be described more fully below, is formed at the periphery of plate. Surround 29 is also metallized to provide an electrical terminal or electrode-which is connected by suitable conductive leads to the electronic circuitry of any known suitable type, which is generally indicated as 37 in FIG. 1, and which may be mounted on the underside of bulkhead 15 as is well-known in the art.
A polarized dielectric film or electret 28, in one embodiment is of polytetrafluoroethylene commonly available under the trademark Teflon, or any such similar material. Electret 28 is mounted on a metal backplate 23. Backplate 23 includes suitable protrusions or posts 44 on which the diaphragm plate 20 rests. to thereby properly position the plate 20 relative to the electret 28. Backplate 23 includes apertures 34 to permit the passage of sound vibrations to cavity 25.
Plate 23 further includes a downwardly extending flange 42 which mounts on bulkhead 15. The electronic circuitry generally indicated as 37 is connected through lead 32 to the surround 29 and through lead 36 to the-backplate 23 as is known in the art.
As is also known, as the diaphragm assembly 19 is vibrated in response to an acoustical or sound wave, an electrical signal is developed in the electronic circuitry 37; Likewise, an electrical signal developed in the electronic circuitry is coupled through leads 32 and 36 to develop a signal causing diaphragm plate 20 to vibrate and provide an acoustical signal output.
Since the diaphragm plate 20 will vibrate between the protrusion supports 44, the system effectively comprises a diaphragm formed of a large number of miniature diaphragm segments which are mechanically stiff.
The surround 29 provides a compliant perimeter for the diaphragm plate 20. Surround 29 which is essentially the same thickness as diaphragm plate 20 is formed at the periphery of the diaphragm plate 20 and bends upwardly out of the planeof the diaphragm plate effecting a roll or corrugation 38 and then extends downwardly to form a foot or flange 40 which is affixable to the bulkhead 15. The surround 29 locates the diaphragm plate 20 in proper position on backplate 23. The surround 29 prevents ready passage of sound from cavity 27 to cavity 25.
In the prior art, properly stressing a rectangular membrane diaphragm is difficult because of the inherent geometry of a rectangle. Note that the present construction eliminates this difficulty since the surround 29 properly locates the position of the diaphragm plate 20 and the stressing of the diaphragm plate is eliminated.
In FIG. 4, the diaphragm assembly 19A has the diaphragm plate 20 metalized as at 24 and 24A on both its lower and upper surfaces to provide additional temperature, humidity and aging stability. It has been found that if the diaphragm plate 20 is metalized on one side only, it may tend to curl due to changes in temperature, humidity and aging. However, if the diaphragm plate 20 is metalized on both sides, the metal coating on the opposite sides has compensating dimensional changes which tend to maintain the diaphragm plate in its initial plane. The surround 29A of FIG. 4 is similar to surround 29 of FIG. 3 and includes a roll portion 38A and a foot or flange 40A.
FIG. 5 shows a third embodiment of the inventive diaphragm construction wherein the diaphragm is formed of laminations, but otherwise, the construction is similar to FIGS. 3 and 4. In FIG. 5, the surround 29B is comprised of lamination sheet or sheets 50 and metalizing coating 51. Surround 29B is relatively thinner than the plate portion of the diaphragm comprised of the additional lamination sheet or sheets 52. Lamination sheet 50 may be of Mylar and sheet 52 may be of metal. As in FIG. 3, the surround 29B includes a roll portion 388 and a foot or flange 40B. The surround 29B of FIG. 5 is utilized where a relatively more flexible surround is desired.
FIG. 6 shows another embodiment of the inventive diaphragm construction wherein the diaphragm assembly 19B comprises a polarized dielectric film or electret metal coating 24B is formed on the upper surface of film 2013. The diaphragm assembly 198 includes'a surround 29C which is similar to surround 29 and includes roll portion 38C and foot or flange 40C. As in FIG. 3 and 4, the surround 29C of FIG. 6 is essentially the same thickness as the plate portion 208.
While the invention has been particularly shown and described with reference to a preferred embodiment thereof, it will be understood by those skilled in the art that various changes in form and details may be made therein without departing from the spirit and scope of the invention.
1. A diaphragm assembly for an electret transducer comprising in combination, a central vibratable plate portion, a compliant surround on the periphery of said plate, a backplate having spaced supports on its surface on which said plate rests, said surround locating said diaphragm plate in proper position on said backplate and enabling the operation of said plate to be independent of temperature, humidity or aging parameters.
2. An apparatus as in claim 1 wherein said electret transducer includes at least two principal sound cavities and said surround acoustically seals one cavity from the other cavity.
3. An apparatus as in claim 1 wherein said surround comprises a roll portion which extends upwardly then extends downwardly from the plane of said plate and a foot for providing attaching means.
4. An apparatus as in claim 1 wherein said plate comprises a dielectric sheet material, and a metalizing coating is on the surface adjacent said supports.
5. An apparatus as in claim 1 wherein said plate comprises a sheet of dielectric material, and a metalizing coating is on both sides of said sheet.
6. An apparatus as in claim 1 wherein said plate and said surround comprise laminated sheets with the surround being of relatively thinner cross-section than said plate.
7. An apparatus as in claim 1 wherein said plate portion comprises an electret.