As a lost-cost and easily-manufactured recording device that offers good performance, electret microphones are one of the most commonly-used microphone types for all manner of devices. Electret microphones enable communication and recording in devices including telephones, smartphones, desktop computers, tablets, headsets, high-performance recording systems, hearing aids, megaphones and more. Due to their proliferation, almost everyone uses this type of microphone on an everyday basis.
An electret microphone uses a ferroelectric material, an electret, that has been permanently charged to be polarized with a positive and negative side. Due to this permanent charge, electret microphones do not need an external power source to polarize the electret plate. As in other condenser microphones, the electret plate creates a capacitor with variable capacitance with its movement in relation to the other plate. That variable capacitance is used to generate the recorded electrical signal.
Gerhard Sessler and Jim West invented the electret microphone while working at Bell labs in 1962. While this type of microphone started out as a low-quality device, steady progress has been made, to the point where electret microphones are sometimes used in recording studios and live music performances.
See a demo on wiring electret capsules for a binaural microphone: