First-Order Gradient Microphone Based on the 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 −50 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.