Noise cancelling microphone

Abstract

A microphone unit comprises a pair of like electroacoustic transducers each having sound-responsive elements in the form of identical diaphragms which are parallel to each other and mounted in a common housing that is open at at least one end for the reception of desired sounds in a direction perpendicular to the diaphragms. The diaphragms are spaced apart a distance no greater than one-quarter of the shortest wavelength of the range of frequencies of the noises to be cancelled; and the housing has further apertures that open into the housing in directions parallel to the planes of the diaphragms, these further apertures being disposed at least one on each side of each diaphragm and the interior ones of said further apertures being spaced apart from their associated diaphragm a distance no greater than one-eighth of the shortest wavelength of the range of frequencies of the noises to be cancelled. The noise sources and channels that the microphone may be exposed to are 1. air-borne ambient noises; 2. air-borne noises due to reflection and echoes; 3. direct vibration transmitted to the microphone housing through non-air channels by contact and/or mounting; and 4. microphony effects of the housing, that is, vibration produced by the housing due to all air-borne sources. Air-borne ambient noises, direct vibrations, and microphony noises will be cancelled by the arrangement of the transducers; while echoes, and reflections, as well as ambient noises, will be attenuated by the presence of the further apertures.