Non-directional electret microphone with an air passage to balance pressures on opposite sides of the diaphragm

Claim 1

A non-directional capacitive microphone assembly comprising: (a) a generally cylindrical casing of electrically conductive material, said casing having an upper end closed except at least one sound hole, and a groove radially extending from said sound hole to the corner thereof in the inner surface of said closed upper end thereof; (b) an electret diaphragm disposed in said casing in a facing relation to said sound hole to receive sound pressure through said sound hole, a front surface of said diaphragm being electrically connected to said casing; (c) an electrode plate disposed in said casing and spaced apart from a rear surface of said diaphragm; (d) a closure means for closing a lower end of said casing air-tightly; (e) an electronic circuit means connected to said casing and said electrode plate; (f) a ring-like spacer of electrically insulative material, said spacer being disposed between said diaphragm and said electrode plate to provide an air space therebetween, said spacer being formed at least in an inner portion thereof with a cut-away portion; and (g) an air passage for providing an air communication between a front side space and a rear side space of said diaphragm to balance air pressures therein, wherein said air passage is defined by said groove, the annular space defined by the inner surface of said casing and the peripheral surface of said diaphragm, the annular space defined by the inner surface of said casing and the surface of said spacer, and the annular space defined by the inner surface of said casing, the outer side surface of said electrode plate and said cut-away portion of said spacer.