A device that converts sound waves into analogous electrical waves. Commonly called a "mic" (pronounced "mike"), it contains a flexible diaphragm composed of film or foil that vibrates as it makes contact with the sound. The diaphragm movement modulates an electrical current by various methods. In a carbon mic, used in telephones for more than a hundred years, the diaphragm alters the pressure in carbon grains, changing its resistance.
In a condenser mic, also called an "electrostatic mic" or "capacitor mic," the diaphragm changes the capacitance between itself and a metal plate, both acting as electrodes. The widely used electret mic has a charged dielectric between the electrodes that generates voltage.
Crystal and Dynamic Microphones
Crystal microphones use a piezoelectric diaphragm that produces voltage when subjected to the sound waves (mechanical pressure).
Dynamic mics, which are like speakers in reverse, use a diaphragm attached to a movable coil that generates voltage as air moves the coil between the poles of a magnet.
Unidirectional shotgun and cardioid mics aimed at a sound source eliminate much of the ambient noise, whereas omnidirectional microphones capture everything in the surrounding environment. The cardioid name comes from its heart-shaped pickup pattern. In the past, bidirectional mics were used for interviews; however, two unidirectional mics are commonly used instead.