A NON-DIRECTIONAL MICROPHONE 



411 



combiiiini; the known effects for a sphere and flat plate shown in h'i^. 6.* 

 Allliout^h the sphere is twice the diameter of the circular plate, it is seen 

 that it has the larger effect only at the lower frequencies. The arrows 

 indicate the probable effect to be taken for that of the flat-faced sphere. 

 Although this result applies strictly only at a point at the center, 

 measurements ■' have shown that up to 15,000 cycles for a one and one- 



a^°^° 



100 500 1000 



FREQUENCY IN CYCLES PER SECOND 



5000 10,000 20,000 



Fig. 6— Measured diffraction effect of a lYi' sphere, IJs" circular plate, and 

 acoustic screen and derivation of diffraction effect of 630-A microphone without 



eighth inch circular disc there is little variation over an area comparable 

 to the effective area of the microphone diaphragm. Hence, the diffrac- 

 tion effect derived in this manner is added to the computed contour 

 pressure response (see Appendix A) to obtain the theoretical field 



* The effect at + 60° incidence has been shown as more significant since the 

 diffraction for + 90° is small. The latter effect corresponds to the optical bright spot 

 at the center of the disc on the side away from the light source. This effect occurs 

 over such a small area for angles very close to + 90° that it is of no practical use m 

 this case. However, this does account for the -f 90° response of microphones often 

 being higher than the + 60° or ■\- 30° response. 



