A Century of Microphones*
B. B. BAUER
Stamford, CT 06905,
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 fiveare 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 aredescribed. 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 usc in space communication.
the microphone. This paper is intended to provide a record of the basic contributions made during that time as well as to survey the engineering principles employed in thepresent-day microphones. A brief look into the future will also be attempted. I PLAN OF THIS pApER From the scientific point of view a microphone may be designed to sense any of the manifestations of the sound wave and to convey it to a transducer which will transform it into electrical energy. A sound wave is accompanied by the presence of an alternating excess pressure called the sound pressure p; theparticles of air are subject to a to-and-fro motion which may be described by their velocity u, and since the medium 0 INTRODUCTION As a sensor which transforms sound into an energy form suitable for amplification and transmission, a microphone is among the most common and useful technological servants of mankind. At this writing, a century of effort has been devoted to inventing and perfecting * ©1962 IRE (now IEEE). Reprinted by permission from Proceedings of the IRE, vol. 50, pp. 719-729 (1962 May). 246 follows the adiabatic chang e in temperature law, there exists an alternating as well as corresponding changes
in density, dielectric constant, magnetic susceptibility, and index of refraction. This paper is confined to those microphones in which the sound pressure or soundpressuregradient are transformed into a force F by use of a diaphragm which, together with an associated electromechanical transducer, is set into motion resulting in generation of electricity. employed in earliest microphones, This is the method and it is virtually
d.Audio Eng.Soc., ol.35,No.4, 1987 V April
CENTURY OF MICROPHONES
the universal method for microphone operation today,Because of their importance to a proper understanding ofmicrophones, briefdescriptionsoftypicaldiaphragms and their interaction with the medium have been ineludedin this paper, Some of the other functions of a sound wave that have found significant but limited application in microphones are 1) the combined action of the particle velocity and the alternating temperature upon a heated fine wire 1 and 2)the combined action of pressure and particle velocity upon a cloud of ions. 2'3 Other possibilities have been considered: the change in dielectric constant or magnetic susceptibility of the air could be used to modulate the frequency of an oscillator; 4 the varying refractive index may be caused to modulate a light beam, 5 for example. Some of these functions may hold a key to microphone...