Abstract
Condenser transducers made with thick wax electrets were first suggested more than 40 years ago by Nishikawa and Nukiyama [Proc. Imp. Acad. (Tokyo) 4, 290 (1928)]. Electret transducers are now in wide use after the introduction of foil electrets (charged polymer films about 1 mil thick) with long charge lifetimes [G. M. Sessler and J. E. West, J. Acoust. Soc. Amer. 34, 1787 (1962); 40, 1433 (1966)]. The charge of the electret replaces the dc bias required in conventional condenser transducers. Charging of the foil is possible with a variety of methods based on thermal, corona-discharge, or electron-beam techniques. Charge retention is particularly good on halocarbons (Teflon FEP, Teflon TFE, Aclar, etc.), where characteristic decay times under normal environmental conditions are measured in terms of decades. Electret transducers have been built as microphones with nondirectional and directional characteristics, as microphone arrays, as earphones, and as electromechanical transducers. Especially electret microphones found widespread practical applications during the past years. Their advantages are excellent frequency response, low distortion, small vibration sensitivity, ruggedness, and low cost.
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