A NON-DIRECTIONAL MICROPHONE 



413 



The variables in a screen of this type are its diameter, impedance, 

 and distance from the microphone grid. It is a combination diffraction 

 and imi)edance screen ; for part of the sound is attenuated by passing 

 directly through the screen while the rest is diffracted. The proportion 

 between the two is a function of the impedance of the screen and of the 

 ratio of its diameter to the wave-length of the sound. At lower fre- 

 quencies most of the sound coming from the top is bent around the 

 screen while at higher frequencies more of it travels directly through 

 and becomes attenuated. For sound coming from the side, the screen 

 has little effect. When sound comes from the bottom some of it is 

 reflected onto the face of the microphone. The acoustic screen thus 

 makes the instrument non-directional in its response characteristics. 



General Design 



Besides these changes designed primarily to reduce the directional 

 effects, extensive changes were made in the internal construction and 

 arrangement of the microphone to make the response more uniform and 

 to extend the frequency range. The general construction is shown in 

 Fig. 8. The desirability of making the diaphragm as small as possible 



PROTECTIVE 

 SCREEN 



DIAPHRAGM 

 AND COIL 



ACOUSTIC 

 RESISTANCE 



Fig. 8 — Simplified cross-sectional view of the non-directional microphone. 



has been pointed out in the discussion of microphone diftVaction. 

 However, decreasing the size rapidly reduces the sensitivity which is 

 proportional to the area of the diaphragm and the flux density of the 



