A Century of Microphones

Of the various manifestations of a sound wave, the action of pressure on a diaphragm still is the universal means for detecting the presence of sound. The diaphragm actuates a transducer converting its motions into equivalent electrical waves. Innumerable transducers have been tried, but five are pre-eminent: 1) carbon, 2) condenser, 3) piezoelectric, 4) moving conductor, 5) moving armature. Important microphone improvements during the late twenties and the thirties have come about as a result of application of equivalent circuit analysis to acoustical structures. The principle of pressure microphones, pressure gradient microphones, combination microphones and phase shift microphones are described. Each of these has found an important niche in modem microphone applications. A small number of important applications require super-directional microphones. Here three approaches are used: 1) reflectors, refractors and diffractors, 2) line microphones, 3) higher-order combination microphones. In the future, improvements in design of directional microphones will continue. Wireless microphones are bound to increase in popularity. New methods of transduction based upon solid-state technology appear to be imminent. Unconventional methods of sound pickup may find wide use in space communication.