406 



BELL SYSTEM TECHNICAL JOURNAL 



The directivity of pressure type microphones is caused by two factors: 

 (a) the variation of the diffraction effect with frequency and with the 

 angle of incidence of the sound wave, and {h) the decrease in pressure 

 due to phase shift which occurs when the direction of the progressive 



Fig. 1— 63U-A 



with deskstand. 



sound wave has a component parallel to the plane of the diaphragm. 

 Each is also a function of the dimensions of the microphone relative to 

 the wave length of the sound, and in the instruments of the size dis- 

 cussed in this paper the effects become large only at frequencies above 

 1000 cycles. Directivity might be a\'oided, therefore, if the micro- 

 phone could be made small enough ; but calculation shows that to make 



