Condenser and Carbon Microphones — ^Their Construction 



and Use * 



By W. C. JONES 



Of the numerous microphones which have been developed since Bell's 

 original work on the telephone, only two are used extensively in sound 

 recording for motion pictures, namely, the condenser microphone and the 

 carbon microphone. 



The condenser microphone was first proposed in 1881 but owing to its 

 low sensitivity was limited in its field of usefulness until the development 

 of suitable amplifiers. In 1917, K. C". W'ente published an account of the 

 work which he had done on a condenser microphone having a stretched 

 diaiihragm and a back plate so designed as to introduce an appreciable 

 amount of air damping. The major portion of the condenser microphones 

 used today in sound recording embody the essential features of the Wente 

 microphone. Marked progress has, however, been made in the design and 

 construction of these instruments with the result that they are not only more 

 sensitive but also more stal)le. The factors which contribute to this im- 

 provement are described in detail in this paper. Recently a number of 

 articles ha\e appeared in the technical press calling attention to certain 

 discrepancies between the conditions under which the thermophone calibra- 

 tion of the condenser microphone is made and those which exist in the studio. 

 The nature of these discrepancies and their bearing on the use of the micro- 

 jjhone are discussed. 



Microphones in which the sound pressure on the diaphragm produces 

 changes in the electrical resistance of a mass of carbon granules interposed 

 between two electrode surfaces have been used commercially since the 

 early days of the telephone. In recent years the faithfulness of the repro- 

 duction obtained with the carbon microphone has been materially improved 

 by the introduction of an air damped, stretched diaphragm and a push-pull 

 arrangement of two carbon elements. This instrument is finding extensive 

 use in sound recording and reproduction fields where carbon noise is not an 

 important factor. The outstanding design features of the push-pull carbon 

 microphone are described in this paper and suggestions made as to the 

 precautions to be taken in its use if the best (luality, maximum life, etc. 

 are to be obtained. 



OF the numerous microphones which have been developed since 

 Bell's original work on the telephone, only two are used exten- 

 sively in sound recording for motion pictures, namely, the condenser 

 microphone and the carbon microphone. It has therefore been 

 suggested that it would be fitting to review at this time the con- 

 struction of these instruments and consider some of their trans- 

 mission characteristics and the precautions which should be exercised 

 in their use. 



Condenser Microphone 



In 1881, A. E. Dolbear ' proposed a telephone instrument which 



could be used either as an electrostatic microphone or receiver. This 



* Presented at Soc. of Motion Picture Engineers' Convention, Oct. 20, 1930; 

 Journal, Soc. of Motion Picture Engineers, Jan., 1931. 



^ "A New System of Telephony," A. E. Dolbear, Scientific American, June 18, 



1881, p. 388. 



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