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 preeminent: 1) carbon, 2) condenser, 3) piezoelectric, 4) moving conductor, 5) moving armature.: Important microphone improvements during the late twenties and thirties have come about as a result of the 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 modern microphone applications.: A small number of important applications require superdirectional microphones. Here three approaches are used: 1) reflectors, refractors, and diffractors, 2) line microphones, and 3) higher order combination microphones.: In the future, improvements in the design of directional microphones will continue. Wireless microphones are bound to increase in popularity. New methods of transduction based on solid-state technology appear to be imminent. Unconventional methods of sound pickup may find wide use in space communications.
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