Special Electret Transducers

Abstract

Electret transducers are capacitive transducers, which depend on a semipermanent electric charge imbedded in the surface of a dielectric to provide the bias field in an airgap. After many years of experimentation in the laboratory, electret microphones have now become commercially available. The purpose of this paper is to highlight some of the properties of electret transducers and electret materials that make them attractive in some special applications. The basic form, an electret diaphragm stretched over a perforated backplate, is a first-order differential microphone. The response of a first-order differential capacitive microphone, the simple construction, and low sensitivity to shock and mechanical vibration make the electret suitable for use in close-talking microphones, for example, in telephone operator's headset. A simple electret microphone can be used to sense the arrival of the wavefront in a shock-wave tube. The stored charge in the electret has been used in experimental key transducers to produce voltage pulses from 5 to 25 V.