MINIATURE CONDENSER MICROPHONE SYSTEM 



453 



obstacle is amenable to mathematical treatment, and was considered 

 theoretically by Raleigh. Quantitative consideration of the effect at 

 a point on a sphere directly in line with the oncoming sound has been 

 given by S. Ballantine.' Fig. 1 shows the effect for other polar 

 angles around the sphere, as computed from Ballantine's equation. 

 Of particular theoretical interest is the curve for a polar angle of 

 180°; that is, the point that should be most completely "shadowed" 

 from the sound. Actually, no shadowing effect appears, the pressure 

 remaining substantially equal to that of the undisturbed field. This 

 case is analogous to that where diffraction of light causes a bright spot 

 to appear in the center of the shadow of a circular disk. The area of 

 this acoustic bright spot is small, as may be seen by the pronounced 

 shadowing of a point only 22^° away. Because of its small area, 

 it is impractical to make use of the effect in microphone design. 



Effect of Phase-Shift in a Plane Sound Wave Traveling 

 Along the Plane of the Diaphragm 

 In Appendix I is given an approximate calculation of the reduction 

 in effective pressure on a circular diaphragm, due to the change in 



Fig. 2 — Loss in effective pressure due to phase-shift in plane sound waves travehng 

 across circular diaphragm. 



phase of a progressive plane wave traveling across it. The effect is 

 plotted in Fig. 2. As might be expected, the reduction becomes 

 ' Loc. cit. 



