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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.
First patent on foil electret microphone by G. M. Sessler et al. (pages 1 to 3) An electret microphone is a type of condenser microphone invented by Gerhard Sessler and Jim West at Bell laboratories in 1962. [24] The externally applied charge used for a conventional condenser microphone is replaced by a permanent charge in an electret material.
West and Sessler optimized the mechanical and surface parameters of the system. [10] Nearly 90 percent of the microphones produced annually are based on the principles of the foil-electret and are used in everyday items such as telephones, camcorders, hearing aids, baby monitors, and audio recording devices among others. [11]
Some condenser microphones can be powered with a 1.5-volt cell contained in a small compartment in the microphone or in an external housing. Phantom power is sometimes used by workers in avionics to describe the DC bias voltage used to power aviation microphones, which use a lower voltage than professional audio microphones.
The operating current of this JFET is typically 0.1 to 0.5 mA and is often referred to as bias, which is different from the phantom power interface which supplies 48 volts to operate the backplate of a traditional condenser microphone. [12] Electret microphone bias is sometimes supplied on a separate conductor. [13]
All microphones made by Schoeps employ traditional (i.e. externally polarized, not electret) condenser transducers, and use small-diaphragm, single-diaphragm capsules, even in microphones which offer two or three different directional patterns. All models introduced since 1973, as well as some models from even earlier, have featured ...