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BELL SYSTEM TECHNICAL JOURNAL 



response shown in Fig. 7 where it is compared with the experimental 

 response. The largest deviation between theoretical and experi- 

 mental values is in the zero degree response at frequencies from 10,000 

 to 15,000 cycles. This difference is attributable to the factor men- 

 tioned earlier, namely, the decrease in effective pressure due to the 

 phase shift of a plane sound wave traveling across the face of the 

 diaphragm. It is of the same order of magnitude as that calculated by 

 H. C. Harrison and P. B. Flanders * for a stretched circular membrane. 

 It is concluded, therefore, that the diffraction effect indicated by 

 arrows in Fig. 6 is representative of that of the actual microphone. 



500 1000 



FREQUENCY IN CYCLES PER SECOND 



5000 10,000 20,000 



Fig. 7 — Comparison of theoretical and experimental field response of a laboratory 

 model of the No. 630-A non-directional microphone without screen. 



Effect of Acoustic Screen 



From the quantitative considerations of the diffraction of the 

 spherical moving coil microphone without screen it becomes clear that 

 if the microphone is to be made non-directional in the vertical plane 

 also, an element must be introduced which compensates for the increase 

 and decrease in field response due to diffraction. 



The screen which was developed for this purpose is a disc 2^2 inches 

 in diameter and made of material having a very high resistance-to-mass 

 ratio. This disc is supported approximately 1/8 inch in front of the 

 microphone grid. The diffraction effect of this screen has been meas- 

 ured in terms of the effect on the face of the microphone. Figure 6 

 gives the effects for sound of 0°, — 90°, and -f 60° incidence and com- 

 pares them with those of the microphone without the screen. From 

 these data it may be seen that the acoustic screen compensates for the 

 microphone diffraction effect, for (1) it has least effect for sound of 0° 

 incidence; (2) it causes a decrease in the — 90° field response; and (3) it 

 causes an increase in the + 60° response. 



