An electret microphone is a type of condenser microphone, which eliminates the need for a polarizing power supply by using a permanently charged material.

An electret

Electret

Electret is a dielectric material that has a quasi-permanent electric charge or dipole polarisation. An electret generates internal and external electric fields, and is the electrostatic equivalent of a permanent magnet. Oliver Heaviside coined this term in 1885...

is a stable dielectric

Dielectric

A dielectric is an electrical insulator that can be polarized by an applied electric field. When a dielectric is placed in an electric field, electric charges do not flow through the material, as in a conductor, but only slightly shift from their average equilibrium positions causing dielectric...

 material with a permanently embedded static electric charge

Electric charge

Electric charge is a physical property of matter that causes it to experience a force when near other electrically charged matter. Electric charge comes in two types, called positive and negative. Two positively charged substances, or objects, experience a mutual repulsive force, as do two...

 (which, due to the high resistance

Electrical resistance

The electrical resistance of an electrical element is the opposition to the passage of an electric current through that element; the inverse quantity is electrical conductance, the ease at which an electric current passes. Electrical resistance shares some conceptual parallels with the mechanical...

 and chemical stability of the material, will not decay for hundreds of years). The name comes from electrostatic and magnet; drawing analogy to the formation of a magnet

Magnet

A magnet is a material or object that produces a magnetic field. This magnetic field is invisible but is responsible for the most notable property of a magnet: a force that pulls on other ferromagnetic materials, such as iron, and attracts or repels other magnets.A permanent magnet is an object...

 by alignment of magnetic domains in a piece of iron. Electrets are commonly made by first melting a suitable dielectric

Dielectric

A dielectric is an electrical insulator that can be polarized by an applied electric field. When a dielectric is placed in an electric field, electric charges do not flow through the material, as in a conductor, but only slightly shift from their average equilibrium positions causing dielectric...

 material such as a plastic or wax that contains polar

Chemical polarity

In chemistry, polarity refers to a separation of electric charge leading to a molecule or its chemical groups having an electric dipole or multipole moment. Polar molecules interact through dipole–dipole intermolecular forces and hydrogen bonds. Molecular polarity is dependent on the difference in...

 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 microphones use PTFE plastic, either in film or solute form, to form the electret.

History

Electret materials have been known since the 1920s, and were proposed as condenser microphone elements several times, but were considered impractical until the foil electret type was invented at Bell Laboratories in 1962 by Gerhard Sessler

Gerhard Sessler

Gerhard M. Sessler is a German inventor and scientist. Sessler invented together with James E. West the foil electret microphone at Bell Laboratories 1962 and the silicon microphone in 1983.He received his Ph.D. from the University of Göttingen in 1959...

 and Jim West

James Edward Maceo West

James Edward Maceo West is an American inventor and acoustician. Along with Gerhard Sessler, West developed the foil electret microphone in 1962...

, using a thin metallized Teflon foil.
This became the most common type, used in many applications from high-quality recording and lavalier

Lavalier microphone

A lavalier microphone or lavalier is a small electret or dynamic microphone used for television, theatre, and public speaking applications, in order to allow hands-free operation. They are most commonly provided with small clips for attaching to collars, ties, or other clothing...

 use to built-in microphones in small sound recording devices and telephones.

Types

There are three major types of electret microphone, 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 condenser 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

Preamplifier

A preamplifier is an electronic amplifier that prepares a small electrical signal for further amplification or processing. A preamplifier is often placed close to the sensor to reduce the effects of noise and interference. It is used to boost the signal strength to drive the cable to the main...

 which does require a small amount of power (often incorrectly called polarizing power or bias). This preamp is frequently phantom power

Phantom power

Phantom power, in the context of professional audio equipment, is a method for transmitting DC electric power through microphone cables to operate microphones that contain active electronic circuitry....

ed in sound reinforcement and studio applications. Other types simply include a 1.5V battery in the microphone housing, which is often left permanently connected as the current drain is usually very small.

External links

• Powering microphones
• Explanation of construction
• Guide for Electret Condenser Microphones
• Rane schematic
• Integrated Circuits for High Performance Electret Microphones — Audio Engineering Society
  Audio Engineering Society
  Established in 1948, the Audio Engineering Society draws its membership from amongst engineers, scientists, other individuals with an interest or involvement in the professional audio industry. The membership largely comprises engineers developing devices or products for audio, and persons working...
  
  Convention Paper
• Modern electret microphones and their applications