Electret Transducing

This invention relates to acoustic transducers, for example, electret condenser microphones.

It is known that extremely precise control of the distance between backplate and diaphragm is very important in an electret microphone. Desirably, variation in the distance from unit to unit should be less than two microns, the distance itself typically being in the range of from 25 to 40 microns.

Techniques heretofore proposed for controlling this distance, or separation, or spacing, have included protrusions (Schmitt U.S. Pat. No. 3,772,133, dated Nov. 13, 1973), a screen (Smulders U.S. Pat. No. 4,160,881, dated July 10, 1979), and directly securing a spacer to a Teflon electret layer on a backplate by heating, the spacer itself being spaced from the backplate by the thickness of the Teflon layer.

In hearing aid microphones the sound entry port is usually on a narrow edge of the microphone rather than the front face, and the diaphragm is supported on a ring or bonded to the backplate. In order to provide an unobstructed path from the entry port to the diaphragm, so called acoustic terminators (e.g, U.S. Pat. No. 4,331,840) or deformations in the housing (e.g., U.S. Pat. No. 4,160,881) have been used.

The electret microphones used in hearing aids have used hybrid rather than monolithic preamplifiers, because only the former were able to provide sufficiently low noise, low current drain, and low operating voltage (pinch-off voltage 0.3 to 0.7 volt). Recently, monolithic JFET preamplifiers have been proposed for hearing aids.

In one aspect our invention features applying high pressure and increased temperature to a backplate/electret layer/peripheral spacer sandwich to cause the electret layer material to flow out of the region between the spacer and the backplate so that the spacer is virtually bottomed out on the backplate. The high pressure permits a sufficiently thin layer of electret such that any variation in thickness is insignificant in relation to overall spacing, and provides a burr-free spacer surface, thereby providing accurately reproducible spacer height. The effective spacer height is the difference between height of the spacer minus thickness of electret layer.

In preferred embodiments the electret layer is made of Teflon 25 microns in thickness; the backplate has a protrusion on the same side as the spacer of about the same height; the spacer is 50 microns in thickness (most preferably greater than 40 microns); the thickness of material between the spacer and the backplate is less than 2 microns (most preferably 1 micron); so that electret:diaphragm spacing is 26 microns.

In another aspect the invention features supporting the diaphragm on one side of a peripheral ring that includes support members extending from the opposite side so as to space the ring from the housing wall at the same time that an unobstructed path is provided to the diaphragm, which makes possible providing the entry port in either an edge or a bottom wall of the microphone housing.

In preferred embodiments the support members are a pair of parallel bars extending between and spaced from opposite portions of the ring; the parallel bars are spaced from each other by a distance greater than the width of the path from the entry port to the region opposite the diaphragm; and the housing for the electret transducer has a side opening.

In another aspect the invention features an electret transducer with a preamplifier including a PMOS metal-oxide-semiconductor transistor. The preamplifier is reliable and simply and inexpensively made. In preferred embodiments the substrate supporting the transistor also includes a polysilicon gate, a polysilicon gate bias resistor and a source resistor.

Other advantages and features of the invention will be apparent from the following description of a preferred embodiment thereof and from the claims.

The preferred embodiment will now be described.

FIG. 1 is a perspective view of an electret transducer according to the invention.

FIG. 2 is a vertical sectional view, taken at 2--2 of FIG. 1, of the FIG. 1 transducer.

FIG. 3 is an exploded perspective view of the FIG. 1 transducer.

FIG. 4 is a partial view of a backplate/electret/ spacer component of the FIG. 1 transducer along with an enlarged view of a portion thereof.

FIG. 5 is an electrical schematic of a preamplifier of the FIG. 1 transducer.

Referring to FIGS. 1 through 3, there is shown transducer 10 including lower housing 12 and cover 14 in which the transducer components are contained. They include diaphragm support 16, diaphragm 18, 50 micron-thick spacer ring 20, 25 micron-thick Teflon electret 22, metal backplate 24, preamplifier 26, and printed circuit board 28. Lower housing 12 has entry port 30 extending from one side thereof and connected to the region below diaphragm 18 via passage 32 between lower parallel bars 34 of diaphragm support 16, which bars space diaphragm 18 above lower housing wall 36. Printed circuit board 28 is supported by the lower surface of cover 14 and has contact 39 making electrical connection with backplate 24 and contacts 38, 40, 41 connected to one of spaced copper soldering terminals carried on insulating material (indicated as a group at 42) adhered to housing 12. Backplate 24 has holes 44 to the region between it and diaphragm 18 and protuberance 47 of height to just touch diaphragm 18.

Referring to FIG. 5, PMOS preamplifier 26 includes polysilicon p-channel metal-oxide-semiconductor transistor 56 manufactured according to CMOS technology. The source of transistor 56 is directly connected to signal contact 40 and connected through 10-20K ohm source resistor 58 on semiconductor substrate of transistor 56 to power source lead 41. The gate of transistor 26 is connected to polysilicon 10.sup.9 to 10.sup.10 ohm gate resistor 60, and by contact 39 to backplate 24, shown diagrammatically in FIG. 5 as part of variable capacitor 62, representing the electret/diaphragm combination. As indicated in FIG. 5, diaphragm 18 is connected to ground 38, as are the drain of transistor 56 and gate resistor 60.

Backplate 24, Teflon electret layer 22, and spacer ring 20 are assembled together by applying very high bonding pressure (about six kilograms per square millimeter) and elevated tempeature (320.degree. C.) to a sandwich of sheets carrying pluralities of the components spaced from adjacent components by breakout tabs (not shown). Sufficient pressure is applied to cause the Teflon material to be forced out of the region between ring 20 and backplate 24 so that approximately 1 micron thick layer 46 remains, the displaced Teflon appearing as ridge 48 (exaggerated in enlarged portion of FIG. 4) near spacer ring 20. The use of the high pressure acts to provide otherwise difficult adhesion of spacer 20 to Teflon, and provides a burrfree surface for spacer 20, resulting in an accurately reproducible spacer height.

Diaphragm 18 is secured to peripheral ring 50 of diaphragm support 16 by adhesive that is applied to the upper surface of ring 50 and cured after diaphragm 18 has been placed under tension and brought into contact with the adhesive. Backplate spacer subassembly 52 is then bonded to diaphragm subassembly 54 using a very thin bead of low viscosity adhesive to avoid significantly increasing the spacing between diaprhagm 18 and backplate 24.

In use, sound waves enter through port 30 and pass into the region opposite diaphragm 18 with little attenuation of the higher frequencies owing to the use of spaced parallel bars 34. Bars 34 rest securely against lower housing wall 36 and also add to the rigidity of diaphragm support 16.

The sound waves cause variations in the distance between diaphragm 18 and electret layer 22, varying the capacitance of the resultant variable capacitor 62 (FIG. 5) (18, 22), providing a signal amplified by transistor 56 provided over contact 40. Spacer 20 provides accurate spacing of diaphragm 18 from backplate 24, resulting in desirably low dispersion in the sensitivity level, a problem with electret microphones.

Transistor 56 has low noise, low current drain, and low operating voltage. Its pinch-off voltage (V.sub.p) is between 0.4 and 0.6 volt; its current drain at 1.3 volts is 20-30 microamps; its noise level is approximately 4 microvolts average "A" weighted; its input capacitance is 4 pF, providing with the electret's 3-4 pF driving capacitance a desirable signal-to-noise ratio of 24 dB. Preamplifier chip 26 has good resistance to high temperature and high humidity.

Other embodiments of the invention are within the scope of the following claims. E.g, the entry port could be provided through lower housing wall 36.