The heart of the Electret Microphone or Electret Condensor Microphone (ECM) is a thin polymer film, permanently charged on one of its surfaces and often referred to as an electret film. For the charge to be permanently stored, the film material must have an exceedingly high resistivity. Such high resistivities are found in some polymer materials, for example fluoroethylenepropylene (FEP), commercially known as Teflon FEP.
In the first implementation of polymer ECM’s, suggested by Gerhard M. Sessler and James E. West in 1962  and shown in Fig. 1, the electret film serves as the membrane of the microphone and also carries an electrode in the form of a metal layer on its outer surface. The membrane is separated by an air gap from a metal back plate representing the second electrode. Alternately, the electret may be cemented to the back electrode; in this case, a membrane consisting of an uncharged material is used.
If sound waves excite the membrane to vibrations the dc-electric field in the air gap, which is due to the electret charges, causes a small ac-voltage between the two electrodes. This voltage is proportional to the sound pressure and serves as the output signal of the microphone.
EMs have been the predominant microphone type from the 1970’s to the early 2000’s. Their advantages are simple design, low vibration sensitivity, possibility of miniaturization, and low cost. Typical commercial electret microphones are shown in Fig. 2. In 2010, the worldwide annual production totaled about 2.5 billion and thus accounted for almost 80 percent of the entire microphone market. Closest runners-up were silicon condenser (MEMS) microphones which now dominate the smartphone market. EMs are still being used in mobile and stationary phones, in other communication devices, in camcorders, audio recorders, hearing aids, toys, hi-fi and studio setups, and in many other applications .
•  “Self-Biased Condenser Microphone with High Capacitance” by G. M. Sessler and J. E. West, in Journal of the Acoustical Society of America, Vol. 34, pp. 1787-1788 (1962).
•  R. Lerch, G. M. Sessler und D. Wolf, „Technische Akustik“, Springer, Berlin, 2009