1. A microphone that has an electret transducer capable of converting audio waves into electromagnetic waves.
2. A microphone consisting of an electret transducer in which the foil electret diaphragm is placed next to a perforated, ridged, metal or metal-coated backplate, and output voltage, taken between diaphragm and backplate.

It is proportional to the displacement of the diaphragm or a thin, flexible sheet that can be moved by sound waves, as in a microphone, or which can produce sound waves when moved, as in a loudspeaker.