The working principle of an electret condenser microphone is that the diaphragm acts as one plate of a capacitor. Vibrations produce changes in the distance between the diaphragm and the back plate. ... This change in voltage is amplified by the FET and the audio signal appears at the output, after a dc-blocking capacitor.

Do electret microphones need power?

While the electret condenser microphone doesn't need a power supply to provide polarizing voltage, an FET impedance matching circuit inside the microphone does require some power. This may be supplied by a small low-voltage internal battery or by an external “phantom” supply.

Are electret microphones any good?

A good back-electret mic can perform every bit as well as a traditional capacitor design, and in some cases a little better, though the electrical charge sealed in the electret material can leak away very slowly over a period of several decades, resulting in a slight decrease in sensitivity.