174 



MICROPHONES 



tion. For a limited frequency range of reproduction the nonlinear dis- 

 tortion is not particularly objectionable. 



A response frequency characteristic of the microphone shown in Fig. 9.1 

 is shown by the graph. The diaphragm of this microphone is a circular 

 plate supported at the edge, see Sec. 3.5. Below the fundamental reso- 

 nance frequency the displacement is proportional to the pressure. Since 

 the change in resistance of the carbon button and the resultant developed 

 voltage is proportional to the displacement, the output will be independent 

 of the frequency below the fundamental resonance frequency. These 

 observations are supported by the response frequency characteristic which 



CMSfMS 



EQUIVALENT 



O-30 



CROSS -SECTIONAL VIEW 



Fig. 9.2. Cross-sectional view and equivalent circuit of an improved single button carbon 

 microphone. The open circuit voltage response characteristics are shown by the graph. A. 

 Response in free space. B. Response for constant sound pressure on the diaphragm. Dots 

 computed from equivalent circuit. (After Jones.) 



depicts uniform response in the low frequency range below the funda- 

 mental resonance frequency. In the region of the resonance frequency 

 the output is accentuated. In the range above the resonance frequency 

 the response falls off rapidly with frequency in a series of peaks and dips 

 which indicate vibrations of the diaphragm or plate in the various modes. 

 See Sec. 3.5. 



A new type of single carbon button microphone^ has been developed in 

 which the response is quite uniform over a wide frequency range. (Fig. 

 9.2.) The conical diaphragm is made of a thin aluminum alloy. At low 

 frequencies the diaphragm vibrates as a single unit. However, at the 

 higher frequencies it is necessary to consider it to be made up of three sepa- 



1 Jones, W. C, Jour. A.I.E.E., Vol. 57, No. 10, p. 559, 1939. 



