An Efficient Miniature Condenser Microphone System * 



By H. C. HARRISON and P. B. FLANDERS 



It has been shown recently that microphones and contiguous amplifiers 

 distort the sound field in which they are placed by reason of their size and 

 the cavity external to the diaphragm of the microphone. For frerjuencies 

 such that the size is large compared to the wave-length of perpendicularly 

 incident sound, reflection causes the actuating pressure to be double that 

 which would exist in the undisturbed field. If the direction of the incident 

 sound be along the plane of the diaphragm, the increase of pressure due to 

 reflection is not as great; but there may be a substantial reduction in 

 effective pressure due to differences in phase across the diaphragm. In 

 addition, cavity resonance produces an increase of pressure at frequencies 

 usually within the working range of the microphone. 



This paper describes a laboratory model of a Wente-type condenser 

 microphone of high efficiency and an associated coupling amplifier which 

 are of such small size that reflection and phase-difference effects are of 

 negligible importance within the audible frequency range; while the cavity 

 is so proportioned that its resonance effect is an aid rather than a detriment 

 to uniformity of response in a constant sound field. 



SEVERAL writers ^ 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. ^ It is a function of the direction of the sound 

 with respect to the diaphragm.^ The distortion due to cavity reso- 

 nance is substantially independent of direction and depends mainly 

 on the relation between the dimensions of the cavity and the wave- 

 length 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 "* or else be so 

 proportioned that resonance occurred at frequencies above the reso- 

 nance 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., May 3, 1932. 



lA. J. Aldridge in P. 0. E. E. Jour., Oct., 1928, pp. 223-225; S. Ballantine in 

 Phys. Rev., Dec, 1928, pp. 988-992; VV. West in /. E. E. Jour., 1929, pp. 1137-1142. 



^Phys. Rev., March, 1922. 



3L. J. Sivian in B. S. T. J., Jan., 1931 pp. 96-116. 



^ S. Ballantine in " Contributions from the Radio Frequency Laboratories," 

 No. 18, April 15, 1930. 



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