Unidirectional Condenser Microphone

A miniature unidirectional condenser microphone includes a diaphragm dividing a housing into first and second chambers

. Each chamber communicates through a sound inlet port to the atmosphere. One of the sound inlet ports is covered by a sheet of porous, non-absorbent, hydrophobic thermoplastic material forming an acoustical resistance.

A miniature unidirectional condenser microphone having a fore and aft orientation, said microphone comprising:a miniature housing;electret diaphragm means dividing said housing into front and rear chambers;means in said housing forming a front sound inlet port for said front chamber;means in said housing forming a rear sound inlet port for said rear chamber;

Dimple means internal of said rear chamber projecting inwardly from the wall of said housing and surrounding said rear inlet port for defining a continuous ridge surrounding said rear inlet port and circumscribing an area larger than andincluding said rear inlet port; and

Sheet material supported in spaced relation to said rear inlet port by said ridge and covering the area circumscribed by said ridge for forming an acoustical resistance in the path of communication established by said rear inlet port betweensaid rear chamber and the exterior of said housing.

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 fromstandpoints 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-uniformporosity 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 variesacross 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 individualsmall elements vary in their acoustical resistance. Thus, when incorporated into microphones, they impart diverse performance characteristics.

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. Suchmaterial 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 alsobe 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 themanufacture 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 directlyover 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. Acousticalresistance material is applied over the second port.

by: Emma

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