The discussion of electret microphones here is mostly correct
in that they are a type of condensor microphone and that they
require an excitation voltage, usually between 1.5 and 10 VDC.
All capacitor mirophone capsules are inherently very high
impedence devices, so high that they cannot drive the connecting
cable. Therefore, a preamplifier (in the early days a vacuum tube,
but more recently a FET) is placed in the microphone case to
drive the cable.
A normal condensor microphone requires a fairly high excitation
voltage, so often a small switching power supply is also placed
in the microphone case to convert the "phantom" power supplied
via the cable to high voltage for capsule excitation.
Electret microphones have permanently charged capsules, made
by heating a special dialectric material while it is in a strong
electrostaic field, so there is no need for high voltage excitation
(and the required switching supply). The excitation voltage for an
electret microphone powers the FET amplifier. An electret, in
practice, is a very simple and elegant microphone... It is made up
of just the capsule and a FET transistor.
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