222 



MICROPHONES 



all directions being equally probable, is termed the directional efficiency of 

 a directional microphone. 



In many of the systems described above, determining the directional 

 efficiency becomes a rather cumbersome job. However, the directional 

 efficiencies of the cosine functions are easily determined. A few of these 



DISTANCE 

 GAIN 



ZA 

 3.2 

 3.2 

 ■4.2 



Fig. 9.41. The directional efficiency of microphones having directional characteristics which 

 are various cosine functions. The ratio of energy response of a nondirectional microphone 

 to the energy response of a directional microphone for sounds originating in random direc- 

 tions is termed directional efficiency. The ratio of the distance at which a directional mi- 

 crophone may be operated as compared to a nondirectional microphone is also shown. 



functions are plotted in Fig. 9.41. The directional efficiency as outlined 

 above is also given. For the same ratio of signal to noise, reverberation, etc., 

 the directional microphone may be operated at V directional efficiency 

 times distance of a nondirectional microphone. By means of the charac- 

 teristics shown in Fig. 9.41, the efficiency of other characteristics may 

 be obtained by comparing characteristics which have approximately the 

 same shape and spread. 



9.7. Wind Excitation and Screening of Microphones. — There are 

 three possible sources of excitation which a microphone is subject to when 

 placed in a wind. There may be pressvire fluctuations due to velocity 

 fluctuations present in the wind even though the microphone is absent. 

 There may be pressure fluctuations due to turbulence produced by the mi- 

 crophone in a wind otherwise free from pressure fluctuations, that is, in a 

 wind of uniform velocity. There may be radiation from the first two 

 sources. The effect of the first source may be reduced by screening which 

 takes advantage of the wind pressure distribution over the microphone, the 



