Microphone

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Microphones convert sound waves to an electronic representation that can be recorded, transmitted or otherwise manipulated. They do the conversion in a variety of ways appropriate to the particular application and cost constraints. In general applications, such as converting voice or music, techniques include using the pressure of sound waves in air to move a thin plate.

The plate, in turn, may:

• move a magnet between coils of wire, producing electricity by the interaction ofmagnetic fields with conductors
• compress carbon granules, changing the resistance
• apply mechanical energy to a piezoelectric crystal, which converts pressure directly to electricity; a more general model of a piezoelectric sensor is called surface acoustic wave

Specialized microphones may be used on sound waves in other than air, and, in this application, may be called hydrophones (in water) or geophones (in air). Hydrophones also are called passive sonar.

Microphones that sense the pressure waves of explosions or other physical phenomena are the sensor in a device called a microbarograph; the "micro" implies that the pressure difference is small with respect to atmospheric pressure. Especially at long ranges, the explosive pressure waves may be below the human hearing range.

In other applications, microphone technology, usually with a piezoelectric or semiconductor method, will convert ultrasonic frequencies above the human hearing range.

Surface acoustic wave sensors can be designed to respond to all, or selected parts, of these various acoustic frequency ranges. They can simultaneously filter and generate signals.