An Electret Mic is a type of electrostatic capacitor-based microphone, which eliminates the need for a polarizing power supply by using a permanently charged material.
An Electret Mic is a stable dielectric material with a permanently embedded static electric dipole moment (which, due to the high resistance and chemical stability of the material, will not decay for hundreds of years). The name Electret Mic comes from electrostatic and magnet; drawing analogy to the formation of a magnet by alignment of magnetic domains in a piece of iron.
Electret Mics are commonly made by first melting a suitable dielectric material such as a plastic or wax that contains polar molecules, and then allowing it to re-solidify in a powerful electrostatic field. The polar molecules of the dielectric align themselves to the direction of the electrostatic field, producing a permanent electrostatic “bias”. Modern Electret Mics use PTFE plastic, either in film or solute form, to form the electret.
Electret Mic 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 James West and Gerhard Sessler, using a thin metallized Teflon foil.
This became the most common type, used in many applications from high-quality recording and lavalier use to built-in microphones in small sound recording devices and telephones.
An electret (formed of electr- from “electricity” and -et from “magnet”) is a dielectric material that has a quasi-permanent electric charge or dipole polarisation. An Electret Mic generates internal and external electric fields, and is the electrostatic equivalent of a permanent magnet.
Although Oliver Heaviside coined this term in 1885, materials with electret properties were already known to science and had been studied since the early 1700s. One particular example is the electrophorus, a device consisting of a slab with electret properties and a separate metal plate. The electrophorus was originally invented by Johan Carl Wilcke in Sweden and again by Alessandro Volta in Italy.
The name derives from “electron” and “magnet”; drawing analogy to the formation of a magnet by alignment of magnetic domains in a piece of iron. Historically, electrets were made by first melting a suitable dielectric material such as a polymer or wax that contains polar molecules, and then allowing it to re-solidify in a powerful electrostatic field.
The polar molecules of the dielectric align themselves to the direction of the electrostatic field, producing a dipole electret with a permanent electrostatic bias. Modern electrets are usually made by embedding excess charges into a highly insulating dielectric, e.g. by means of an electron beam, corona discharge, injection from an electron gun, electric breakdown across a gap, or a dielectric barrier.
There are three major types of Electret Mics, differing in the way the electret material is used:
- Foil-type or diaphragm-type
- A film of electret material is used as the diaphragm itself. This is the most common type, but also the lowest quality, since the electret material does not make a particularly good diaphragm.
- Back electret
- An electret film is applied to the back plate of the microphone capsule and the diaphragm is made of an uncharged material, which may be mechanically more suitable for the transducer design being realized.
- Front electret
- In this newer type, the back plate is eliminated from the design, and the capacitor is formed by the diaphragm and the inside surface of the capsule. The electret film is adhered to the inside front cover and the metalized diaphragm is connected to the input of the FET. It is equivalent to the back electret in that any conductive film may be used for the diaphragm.
Unlike other condenser microphones, electret types require no polarizing voltage, but they normally contain an integrated preamplifier, which does require a small amount of power (often incorrectly called polarizing power or bias). This preamp is frequently phantom powered in sound reinforcement and studio applications. Other types simply include a 1.5 V battery in the microphone housing, which is often left permanently connected as the current drain is usually very small .