An Efficient Miniature Condenser Microphone System

E V E R A L writers 1 have recently called attention to the fact that a microphone distorts the sound field in which it is placed by reason of its size and the cavity external to the diaphragm. The distortion due to size was first mentioned by I. B. Crandall and D. MacKenzie in 1922.2 It is a function of the direction of the sound with respect to the diaphragm.3 The distortion due to cavity resonance is substantially independent of direction and depends mainly on the relation between the dimensions of the cavity and the wavelength of sound. If a microphone were to be designed so that it would respond uniformly to sound coming from any direction, it is apparent that first the size would have to be diminished to such an extent that reflection and phase-difference effects became negligible. Secondly, the cavity would either have to be eliminated entirely 4 or else be so proportioned that resonance occurred at frequencies above the resonance frequency of the diaphragm, where the response of the latter was diminishing. Such mutual compensation is possible in a small microphone and the effect is substantially independent of the direction of sound. * Presented before Acous. Soc. Amer., New York, N. Y., M a y 3, 1932. ' A . J. Aldridge in P. 0. E. E. Jour., Oct., 1928, pp. 223-225; S. Ballantine in Phys. Rev., Dec., 1928, pp. 988-992; YV. West in I. E. E. Jour., 1929, pp. 1137-1142. 2 Phys. Rev., March, 1922. 3 L. J. Sivian in B. S. T. J., Jan., 1931. pp. 96-116. 4 S. Ballantine in " Contributions from the Radio Frequency Laboratories," No.