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BELL SYSTEM TECHNICAL JOURNAL 



considerable when the diameter of the diaphragm exceeds the wave- 

 length. Since in normal use, a large proportion of the sound actuating 

 the microphone comes in a transverse direction, the distortion due to 

 this phase-cancellation effect may often be more serious than the 

 distortion due to diffraction. 



Cavity Resonance 

 The effect of a cylindrical cavity on the pressure actuating a dia- 

 phragm is given by equation (5) in Appendix II and is shown graphi- 

 cally in Fig. 3. It is apparent that the larger the ratio of diameter 



Fig. 3 — Gain in effective pressure on circular diaphragm due to cavity resonance. 



to wave-length, the smaller is the resonance effect; and that the greater 

 the ratio of a given diameter to the depth of the cavity, the higher the 

 frequency at which such resonance occurs. There is, of course, more 

 than one resonance. However, for the higher resonances there is a 

 greater ratio of diameter to wave-length with a consequent increase in 

 damping; so that the resonance effect is less pronounced. Compared 

 to the primary resonance, the secondary resonances are, as a rule, 

 of negligible importance. 



Acoustic and Electrical Considerations Affecting the Size 

 OF A Condenser Microphone 

 If diffraction effects for all angles of incidence are to be negligible 

 in a frequency range extending, for example, to 15,000 c.p.s., it is 

 apparent from Fig. 1 that the diameter of the microphone should not 

 be greater than about a tenth of an inch. Assuming that all other 

 dimensions and characteristics remain fixed, including the resonance 

 frequency of the diaphragm and the ratio of dead to active capacity, 

 it can be shown that as the diameter of a condenser microphone 

 diaphragm is decreased, the ratio of generated voltage to actuating 



