microphone, device for converting sound into electrical energy, used in radio broadcasting, recording, and sound amplifying systems. Its basic component is a diaphragm that responds to the pressure or particle velocity of sound waves. The microphone, various forms of which were developed independently c.1877 by inventors Emile Berliner, David E. Hughes, and Thomas A. Edison, was first used as a telephone transmitter. The carbon microphone, which was used in the first telephones and was very popular in telephones until about 1970, contains loosely packed carbon grains. Sound makes the diaphragm vibrate, causing the grains to be compressed and released, thus changing the resistance of the microphone. That can be exploited by an associated electric circuit. Electrostatic microphones, also called condenser microphones, consist of a fixed electrode (the backplate) and a movable electrode (the diaphragm), with an air gap between them. Sound waves impinge on the diaphragm, making it vibrate, and changing the capacitance formed by the two electrodes. Electret microphones, which are the most widely used microphones, have a permanently charged dielectric between the two electrodes and thus generate voltages when the electrodes vibrate. Crystal microphones generate minute voltages by the piezoelectric effect . Both the dynamic microphone and the rarely used ribbon microphone generate voltages by electromagnetic induction . For example, in the dynamic microphone, the diaphragm is attached to a light movable coil that generates a voltage as it moves back and forth between the poles of a permanent magnet.
See G. M. Ballou, Handbook for Sound Engineers (1991).