Self-Biased Condenser Microphones with High Capacitance

Abstract

A special condenser microphone with solid dielectric between the electrodes is described. The system employs metallized dielectric foil (14 mil thick) which is prepolarized. Therefore, no external dc bias is required. The capacitance of a unit 3.8 cm in diameter is in the order of 800 μμF. This high capacitance eliminates the need of a cathode follower, and allows the use of a long connecting cable. The frequency response of this system is flat within ±3 dB between 50 and 15 000 cps. The sensitivity of the system is −56 dBV for a sound pressure of 1 dyn/cm2 (74 dB SPL). The upper frequency limit and the sensitivity are controlled by the shape of the backplate surface. The self-noise of the microphone is considerably below that of low-noise pre-amplifiers. The upper limit of the dynamic range is set by the harmonic distortion. At 140 dB SPL, the harmonic distortion over the entire frequency range is less than 3%.