Electret microphone

[1] The electret's constant charge eliminates the need for the polarizing power supply required for non-electret condenser microphones, though a preamplifier is typically incorporated to boost the audio voltage signal.

An electret is a stable dielectric material with a permanently-embedded electric dipole that persists for hundreds of years due to its high resistance and chemical stability.

Electret materials have been known since the 1920s and were proposed as condenser microphone elements several times, but they were considered impractical until the foil electret type was invented at Bell Laboratories in 1961 by Gerhard Sessler and James West, using a thin metallized Teflon foil.

This preamp is frequently phantom powered in sound reinforcement and studio applications.

Other types include a 1.5 V battery in the microphone housing, which is often left permanently connected as the current drain is usually very small.