Unidirectional condenser microphone - Patent Review 3963881

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CROSS REFERENCE TO RELATED APPLICATION

This application discloses subject matter similar to that appearing in copending Application Ser. No. 364,984, filed May 29, 1973 for Condenser Microphone in the name of Freeman W. Fraim now U.S. Pat. No. 3,895,194.

BACKGROUND OF THE INVENTION

Miniature unidirectional condenser microphones require high quality and very small acoustical resistance elements having predictable resistance characteristics. Resistance elements of porous materials are potentially advantageous from standpoints of performance, cost and ease of assembly. However, potentially useful porous elements, if manufactured in thin sheets to provide the dimensional requirements suitable for a miniaturized microphone, tend to be characterized by a non-uniform porosity per unit area. The production of multiple acoustical resistance elements of miniature size and predictably uniform acoustical resistance characteristics is thus not possible. That is, since an acoustical resistance of said materials varies across the surface of the materials, a relatively large area is required to establish a predictable acoustical resistance; if acoustical resistance elements of sizes required for miniature microphones are fabricated from such materials, the individual small elements vary in their acoustical resistance. Thus, when incorporated into microphones, they impart diverse performance characteristics.

SUMMARY OF THE INVENTION

This invention pertains to a miniature unidirectional condenser microphone having an acoustical resistance of porous thermoplastic material in thin sheet form. The porous thermoplastic material is preferably non-absorbant and hydrophobic. Such material is characterized by a consistent acoustical resistance per unit area over its entire surface. Small resistance elements all having a predictable acoustical resistance characteristic can thus be produced. Porous thermoplastic material can also be manufactured in very thin sheets so that the overall dimensions of the acoustical resistance elements produced therefrom are compatible with a miniaturized microphone.

For an acoustical resistance material of given characteristics, the total acoustical resistance provided in a given microphone is a function of the area of the porous thermoplastic material exposed to incoming sound waves. However, in the manufacture of miniature condenser microphones, it is desirable to have a sound inlet port of a size independent of the acoustical resistance characteristics of the microphone. Normally therefore application of acoustical resistance material directly over the sound inlet port is not a suitable arrangement. According to this invention, there is provided internal of the microphone a device to establish a second port having an area functionally related to the required acoustical resistance. Acoustical resistance material is applied over the second port.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a cut-away, perspective view showing a preferred embodiment of the invention;

FIG. 2 is an exploded perspective view of the apparatus shown in FIG. 1;

FIG. 3 shows a modification of the apparatus of FIGS. 1 and 2;

and FIG. 4 is a schematic illustration showing the microphone of the present invention in combination with a hearing aid.

DETAILED DESCRIPTION OF THE DRAWINGS

Referring to FIGS. 1 and 2, the miniature microphone 10 incorporates a housing 12 comprised of an electrically conductive front member 14 and a rear member 16. In a preferred embodiment, the miniature housing 12 measures slightly less than 0.10 inches in thickness and has a face area of approximately 0.22 .times. 0.31 inches. Within the housing 10 is mounted an assembly 18 comprising a backplate 20, a diaphragm 22 and a board 24 having an electrical circuit thereon.

The backplate 20, formed of injection molded thermoplastic material, has a peripheral ridge 26 extending from an obverse face. The diaphragm 22 is mounted on the ridge 26 and spaced from the backplate by a distance equal to the height of the ridge 26. To avoid contact between the backplate and the diaphragm during operation of the microphone, a pair of posts 28 are provided. The posts approximately equal or slightly exceed the height of the ridge 26. There is formed, between the backplate and the diaphragm, a working gap 30. In the reverse side of the backplate 20 is a recess 32 for receiving the circuit board 24. The configuration of the recessed area conforms substantially to the configuration of the circuit board, except that along one portion thereof a portion of the recess 32 extends beyond the board 24 to form an open area 34. The circuit board 24 is securely bonded to the backplate 22 by an epoxy resin 66. A plurality of holes 36 extend through the backplate and communicate between the working gap 30 and the recess area 32. A system of grooves 38 in the reverse side of the backplate 20 connects the holes 36 and extends into the open area 34. The epoxy resin 66 is positioned not to block the constriction formed by the grooves 38 which connect the holes 36 with the open area 34. A thin coating 40 of electrically conductive material, such as gold, renders the backplate electrically conductive.

The diaphragm 22 comprises an electret 42 having thereon a coating 44 of electrically conductive material, such as gold. The electret is described in U.S. Pat. No. 3,612,778 titled Electret Acoustic Transducer and Method of Making. The diaphragm is mounted on the backplate by firmly securing the electret along the ridge 26 with the coating 44 facing away from the backplate 20.

The circuit board 24 is mounted in the recess 32 with the side bearing electrical components means 46 facing away from the backplate. A relatively smooth side 48 seats within the recess 34. A plurality of leads 50 extend from the circuit board for connection to a terminal board 52 mounted on the housing 12.

The assembly 18 is mounted within the housing 12 to form a first or front air chamber 54 adjacent the front housing member 14 and a rear air chamber 56 adjacent the second or rear housing member 16. The spacing is accomplished by a ring 58 and a resilient gasket 60. The ring 58 is plastic with an electrically conductive coating 62. It establishes electrical connection between the diaphragm and the front member 14. The resilient gasket 60 spaces the assembly 18 from the rear member 16 and loads the assembly so that it is pressed towards the front member 14.

Means 68 extends from the front housing member 14 to form a front sound inlet port 70, communicating with front air chamber 54 through an opening 64 in the ring 58.

During operation of the microphone, the rear air chamber 56 acts as a resonance chamber. It is in fluid communication with the working gap 30 through the holes 36 and the grooves 38. The grooves 38 and the surface 48 of the circuit board 24 form a air passageway which constitutes a constriction in the fluid communication path between the working gap 30 and the rear air chamber 56 to damp the diaphragm 22. The character of the passageway formed by the grooves 38 and the surface 48 determines the amount of damping which occurs. It will be appreciated that the size of the groove as well as its length are the fundamental parameters which may conveniently be designed to provide a desired damping action.

To provide the microphone 10 with unidirectional characteristics, there is in the housing member 16 a rear sound inlet port 74 which communicates with the rear air chamber 56. Mounted along the inner surface of the rear portion 16 is a spacer of sheet material 76 having an opening 78 therein. Opening 78 defines an area larger than the area of the rear port 74 and is superposed thereover. Over the spacer sheet 76, within the rear air chamber 56, a sheet of acoustical resistance material 80 covers the opening 78 and is spaced from the rear sound inlet port 74 by a distance equivalent to the thickness of the spacer sheet. Stated differently, the opening 78 and the spacer sheet 74 form a raised ridge around the inlet port 76 to define an area larger than the area of inlet port and to encompass the inlet port. Other means could be used to define this raised ridge. For example, by reference to FIG. 3, a raised dimple 82 spaces the resistance material 80 from the rear port 74 to define the opening 78. In operation of the microphone, acoustical input acts both upon the front and back of the diaphragm 22. The resistance 80 applies a delay, or phase shift, to the signal incident upon the rear of the diaphragm 22 to provide a unidirectional characteristic. Specifically the microphone 10 has a directional characteristic preferential to the front. By way of example, the unidirectional characteristic may be of cardioid configuration.

The acoustic resistance element 80 must have an acoustical resistance not less than 200 cgs ohms and not more than 5000 cgs ohms, the exact value depending upon the performance characteristic desired. One unidirectional microphone for hearing aid use has an acoustic resistance of approximately 1,000 cgs ohms. Both membranes and sintered plastics provide acceptable resistance elements. One suitable porous membrane is of themoplastic, a copolymer of acrylonitrile and polyvinylchloride on a nylon substrate. It is manufactured by Gelman Instrument Company, 600 South Wagner Road, Ann Arbor, Mich. and sold under the trade name "Acropor". For hearing aid purposes, a suitable microphone includes such a membrane having a 5 micron pore size and a membrane thickness of 0.0045 inches. Membranes having pore sizes in a range of from 3 to 10 microns at not exceeding 0.020 inches in thickness are acceptable. A thickness dimension of 0.015 inches or less is preferred. Also suitable is a polypropylene sheet 0.008 inches thick with a pore size of 10 microns, also manufactured by Gelman Instrument Company. In miniature microphones, especially those not exceeding 0.10 inch in thickness, the thickness dimension of the resistance element is highly important. To maintain a compact and durable structure, the resistance element 80 is mounted internal of the housing 12. There is available only a small space between the inner surface of the rear housing member 16 and components of the electrical circuit board 24. This distance is limited to approximately 0.020 inches.

Sintered thermoplastics such as high density polyethelene and fluorocarbons also form suitable resistance elements 80. High density polyethylenes are those having specific gravity between 0.941 and 0.965. Those having pore sizes not exceeding 70 microns are effective. Pore sizes not less than 10 microns or more than 30 microns are preferred. As with other resistance elements discussed above, thickness should not exceed 0.020 inches, a thickness of 0.015 inches or less being preferred.

All the above materials are substantially non-absorbant and hydrophobic. The non-absorbant characteristic prevents humidity and moisture induced swelling of the resistance material which blocks the pores therein. Similarly, the hydrophobic characteristic avoids filling the pores with water by wicking.

FIG. 4 illustrates schematically the microphone of this invention in a hearing aid. The microphone can provide an electrical output to an amplifier 84 which drives a transducer forming a hearing aid receiver 86. The unidirectional characteristic provides a hearing experience which approximates normal, unaided hearing.

This invention has been described with reference to a preferred embodiment. It will be understood by those skilled in the art that changes may be made without departing from the scope of the invention.

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