A NON-DIRRCTIONAL MICROPHONE 



407 



the effect negligible at 10,000 cycles the instrument would have to be 

 approximately one-half inch in diameter. While a microphone of this 

 size could be built, it is doubtful whether an output level could be 

 obtained which would be adeciuate for i)ublic address, broadcasting, 

 and sound ]iirture use. 



-70 

 -75 

 -80 

 -85 

 -90 



30 



100 500 1000 



FREQUENCY IN CYCLES PER SECOND 



5000 10,000 



Fig. 2 — Field response of the 618-A microphone showing the 

 effect of angle of sound incidence. 



Diffraction plays by far the predominant role up to a frequency where 

 the wave-length is comparable to the diameter of the diaphragm, and 

 the effect of phase shift may be neglected. It is principally diffraction 

 which causes the field response of a microphone to differ from the 

 pressure response, and because of the variation of the effect with angle 

 of sound incidence it is impossible to correct for it by adjusting the 

 pressure response. The only alternative is to attack the diffraction 

 problem directly. 



