Foil-Electret Microphones

Abstract

Electrostatic microphones with foil electrets are described. Foil electrets are solid dielectric materials, such as fluorocarbon, prepolarized in an electrostatic field at an elevated temperature. The use of electrets eliminates the need for an external dc bias. The sensitivity of these microphones depends largely on the surface charge of the electret and is typically between −50 and −60 dBV for a sound-pressure level of 1 μbar. The sensitivity of fluorocarbon electret microphones does not decay over periods of the order of years at room temperature. Extrapolation tn room temperature of results obtained at elevated temperatures indicates the time constant of the sensitivity of fluorocarbon microphones to be between 30 and 103 years, depending on the preconditioning (temperature cycling) of the systems. The relative humidity is of little influence on the time constant. Even at 80°C and high relative humidity, the time constant is still of the order of 100 days. The capacitance per unit area of electret microphones (about 60 pF/cm2) is three times greater than that of air-gap systems. Because resonance frequency and sensitivity do not depend on the diameter of the system, a capacitance much higher than that of air-gap systems can be achieved for a given bandwidth. The free-field frequency response is within ±1.5 dB from 50 to 14 000 Hz when the microphone is terminated by a few megohms.