FIRST-ORDER GRADIENT MICROPHONE BASED ON FOIL-ELECTRET PRINCIPLE - DISCRIMINATION AGAINST AIR-BORNE AND SOLID-BORNE NOISES

Design and performance of a simple first order pressure gradient microphone based on the electrostatic principle using foil electrets are discussed. Such a microphone discriminates against air borne noise as well as against solid borne vibrations transmitted through the microphone case. For air borne sound originating from a spherical sound source positioned close to the microphone, the frequency response is flat within ±3 dB in the frequency range 0.1 to 3 kHz with a sensitivity of about â50 dBV/μbar in the direction of the microphone axis. In a plane sound field, the microphone exhibits the typical first order gradient behavior, i.e., its sensitivity rises proportional to frequency over most of the mentioned frequency range. At 0.1 and 1 kHz, the plane wave sensitivity is 30 and 10 dB, respectively lower than the sensitivity to sound originating from a nearby source, making this microphone a noise discriminating device for close talking applications. Rejection of solid borne vibrations is due to the lightness of the foil electret diaphragm. Compared with electrodynamic and electromagnetic transducers, the foil electret microphone responds 30 to 40 dB less to solid borne vibrations, referred to equal output for air borne sound. © 1969, Acoustical Society of America. All rights reserved.