214 



MICROPHONES 



quency response may be overcome by moving the microphone slightly out 

 of focus. This expedient also tends to broaden the sharp directional 

 characteristics at the high frequencies. 



The directional characteristics of a parabolic reflector 3 feet in diameter, 

 used with a pressure microphone, are shown in Fig. 9.31. It will be seen 



SECTIONAL VIEW 



Fig. 9.31. Cross-sectional view of a parabolic reflector for a microphone. The polar graphs 

 show the directional characteristics. The polar graph depicts the pressure, in dynes, at the 

 microphone as a function of the angle, in degrees. The maximum response is arbitrarily 

 chosen as unity. (After Hanson.) 



that the directivity increases with frequency. For example, the system 

 is practically nondirectional at 200 cycles. On the other hand, the direc- 

 tional characteristic is very sharp at 8000 cycles. 



B. Line Microphones ^^' ^^^' ^^^. — A line microphone is a microphone 

 consisting of a number of small tubes with the open end, as pickup points, 

 equally spaced along a line and the other end connected to a common junc- 

 tion to a transducer element for converting the sound vibrations into the 

 corresponding electrical variations. In the line systems to be considered, 

 the transducer will be a ribbon element located in a magnetic field and ter- 

 minated in an acoustic resistance. Under these conditions the output of 

 the pipes can be added vectorially. 



1. Line Microphone: Useful Directivity on the Line Axis. Simple 

 Line. — This microphone consists of a number of small pipes with the 

 open ends, as pickup points, equally spaced on a line and the other ends 

 joined at a common junction decreasing in equal steps (Fig. 9.32). A 

 ribbon element, connected to the common junction and terminated in an 



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



35-* Mason and Marshall, Jour. Acous. Soc. Amer., Vol. 10, No. 3, p. 206, 1939. 



"■B Olson, H. P., Broadcast News, No. 28, p. 32, July, 1938. 



