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Electret microphone

Electret microphone assemblies include an electret microphone disposed within a housing. An electret is a dielectric that produces a permanent external electric field which results from permanent ordering of molecular dipoles or from stable uncompensated surface or space charge. The electret microphone has a diaphragm and a charged plate. An electret microphone has a permanently charged dielectric (electret) between two parallel metal plates (electrodes), one of which is attached to a diaphragm. Sound vibration vibrates the diaphragm, which vibration generates an electrical signal on the charged plate. This diaphragm moves in response to the pressure or particle velocity of sound waves, thereby changing the distance and, therefore, the capacitance, between the diaphragm and its electrode, or backplate. Often the housing also contains a substrate. The substrate can support an impedance matching circuit for matching the high impedance of the electret microphone with the relatively lower input impedance of an external amplifier. The electret element is enclosed in a housing together with a buffer amplifier, most commonly a JFET stage. A voltage bias source is therefore required for the JFET amplifier and is provided by the equipment the microphone is connected to. Electret microphones are preferred in many applications for their small size, light weight and high output. Electret microphones are commonly used in headsets for telephony, computer telephony and speech recognition applications.

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