The electret mic is a form of capacitor microphone.
However the charge is permanently locked into the diaphragm and backplate, just as magnetic energy is locked into a magnet.
Not all materials are suited to forming electrets, so it is usually considered that the compromises involved in manufacture compromise sound quality.
However, it has to be said that there are some very good electret mics available, most of which are back-electrets, meaning that only the backplate of the capacitor is an electret therefore the diaphragm can be made of any suitable material.
Electret mics do still need power for the internal amplifier. However, this can take the form of a small internal battery, which is sometimes convenient.
Electret mics that have the facility for battery power can also usually be phantom powered, in case the battery runs down or isnt fitted.
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