Moving-Coil Telephone Receivers and Microphones * 



By E. C. WENTE and A. L. THURAS 



A description is given of a moving-coil head receiver and a microphone 

 designed particularly for high quality transmission. The instruments have 

 a substantially uniform response from 40 to 10,000 c.p.s. This uniformity 

 of response has been obtained, without sacrifice of sensitivity, by the use of 

 light mo\'ing parts and the association of special types of acoustic networks 

 with the diaphragm. In practical use the microphone has a sensitivity 

 about 10 db higher than that of the Western Electric 394 Condenser 

 Microphone. 



MOVING-COIL loud speakers are now extensively used in high 

 quality radio-receiving sets and in talking motion picture 

 equipment. The chief advantages of the moving coil over the moving 

 armature driving mechanism are the absence of a static force, con- 

 stancy of force-factor and electrical impedance throughout a wide 

 frequency range, and freedom from non-linear distortion over a wide 

 amplitude range. Because of these advantages it seems obvious that 

 the moving coil structure can also be used profitably in head receivers 

 and microphones where high quality is of prime importance. It has 

 therefore been adopted in the instruments to be described, although 

 some of the principles here formulated can conceivably be applied also 

 to instruments with moving armatures. This paper is concerned 

 primarily with the general principles of design. The more practical 

 phases of the commercial design and construction of the microphone 

 are discussed in a paper by W. C. Jones and L. W. Giles. ^ 



The moving system of a head receiver must, in general, satisfy 

 distinctly different requirements from that of a microphone. In the 

 actual use of the receiver a small enclosed cavity is formed between the 

 ear and the diaphragm. If there is to be no distortion the pressure 

 developed within this enclosure per unit of current in the receiving 

 coil should be independent of frequency, constancy of impedance of 

 the coil being assumed. The pressure depends not only upon the 

 amplitude of vibration of the diaphragm, but also upon the acoustic 

 impedance of the cavity formed by the ear and the receiver. This 

 impedance is such, if the cavity is entirely enclosed, that at low fre- 

 quencies the pressures will be very nearly proportional to the dis- 

 placement of the diaphragm. At higher frequencies it is of uncertain 

 value and varies from ear to ear, but it appears, from unpublished 



* Jour. Acous. Soc. Amer., July, 1931. 



1 "Moving Coil Microphone for High Quality Sound Reproduction." Presented 

 at May 1931 meeting of Soc. of Motion Picture PIngineers, Hollywood, California. 



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