PRESSURE MICROPHONES 



185 



phones are aluminum alloys, Bakelite, styrol and paper. In order to obtain 

 a maximum ratio of conductivity to mass, aluminum is almost universally 

 used for the voice coil. Both edgewise wound ribbon and round wire have 

 been used for the voice coil. See Fig. 7.18. 



The diffraction of sound as a function of the angle of the incident sound 

 by various objects shows that the sphere exhibits the most uniform direc- 

 tional pattern (Fig. 1.5). A spherical case with the diaphragm located on 

 the surface of the sphere seems to be the logical starting point for a non- 

 directional pressure microphone. Referring again to Fig. 1.5, it will be 

 seen that the microphone will show excess response over the range from 

 0° to 60° and will be lacking in response from 120° to 160°. This non- 

 uniform response can be corrected by placing a disk, of semi-transmitting 

 characteristics, of diameter equal to the spherical case directly above the 

 diaphragm and spaced one-fourth inch. Employing this expedient, a non- 

 directional characteristic ^^ is obtained for all frequencies. 



DIAPHRAGM } ^ ^ BOLT 



■ unamrr ^ ^'msif^ OF 



m, "-Ml r„, 

 r^WfU" — 1|— vwv 



CROSS -SECTIONAL VIEW 



1000 

 FREQUENCY 



Fig. 9.7. Cross-sectional view and the equivalent circuit of an inductor microphone. The 

 graph shows the free space open circuit voltage response frequency characteristic. 



2. Inductor Microphone '^'^ {Straight Line Conductor). — The inductor 

 microphone is another example of a moving conductor microphone. A 

 cross-sectional view of this microphone is shown in Fig. 9.7. The dia- 

 phragm rmCuimx of this microphone is " V " shaped with a straight con- 

 ductor located in the bottom of the " V." The equivalent circuit of this 

 microphone is the same as that of the dynamic microphone in the preceding 



" Marshall and Romanow, Bell Syst. Tech. Jour., Vol. 15, No. 3, p. 405, 1936. 

 12 Olson, H. F., U. S. Patent 2,106,224. 



