220 



MICROPHONES 



4. UUradirectional Microphone ^e.se-i^ — Directional microphones employ- 

 ing lines of various types have been considered in the preceding section. 

 These directional characteristics indicated considerable variation with 

 frequency. Experience gained from work on reflectors a few years ago 

 indicated that a directional characteristic which varies with frequency is 

 undesirable, principally due to the introduction of frequency discrimina- 

 tion for points removed from the axis. In addition, the response to re- 

 flected sound is a function of the frequency which alters the reverberation 

 characteristics of received sound. 



From the results of experiments upon directional systems, it appears 

 that a microphone with a small solid angle of pickup would be useful in 

 recording sound motion pictures, in television pickup, in certain types of 

 sound broadcast as, for example, symphony and stage productions, and in 

 many applications of sound re-enforcing. The acoustic lines referred to 

 above seem to be the logical solution of the problem from the standpoint 



PICKUP POINTS- 



/h 



» » t H »i 



XUUUU 



DELAY 



'7rl.iJJ.IJJJJ.n,n,n.n,f^.., 



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PRESSURE GRADIENT ELEMENTS 



Fig. 9.38. Ultradirectional microphone consisting of five units. Units 1, 2 and 3 are of the 

 type shown in Fig. 9.32. Units 4 and 5 are of the type shown in Fig. 9.36. An electrical 

 filter system is used to allocate the output of the units to their respective ranges. 



of size and portability. However, the directional characteristics must be 

 independent of the frequency. This can be accomplished by employing a 

 number of separate lines, each covering a certain portion of the frequency 

 range. It is the purpose of this section to describe an ultradirectional 

 microphone consisting of five separate lines. 



The ultradirectional microphone shown schematically in Fig. 9.38 con- 

 sists of five units. Units 1, 2 and 3 are of the type shown in Fig. 9.32. 

 Units 4 and 5 are of the type shown in Fig. 9.36. An electrical filter sys- 

 tem is used to allocate the outputs of the units to their respective ranges. 



36 Olson, H. F., Jour. Inst. Rad. Eng., Vol. 27, No. 7, p. 438, 1939. 

 36^ Olson and Massa, " Applied Acoustics," P. Blakiston's Son and Co., Philadel- 

 phia. 



