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BELL SYSTEM TECHNICAL JOURNAL 



The directional effect, while diminished somewhat compared with that 

 of the 618-A type (Fig. 2), is still not negligible. It was possible, how- 

 ever, to achieve a fairly uniform response with respect to frequency for 

 sound of 0° incidence, that is, sound arriving in the plane of the dia- 

 phragm. Since in most instances the direct energy arrives in the 

 horizontal plane, uniform, non-directional response for this important 

 plane may be secured by mounting the microphone with the diaphragm 

 in a horizontal position. Still there is a tendency for the response to be 



-75 

 -80 

 -85 

 -90 

 -95 

 -100 

 -105 

 -110 



-75 



30 



500 1000 



FREQUENCY IN CYCLES PER SECOND 



5000 10,000 20,000 



Fig. 5 — Field response of a laboratory model of the 630-A 

 non-directional microphone without screen. 



too high for high-frequency sounds coming down from above, that is, 

 directly toward the diaphragm, and too low for similar frequencies 

 coming from angles very much below the horizontal. 



To determine to what extent diffraction contributes to this residual 

 directivity in the vertical plane let us consider the diffraction of a 

 geometrical shape resembling that of the microphone, namely, a two 

 and one-half inch sphere with a flat face one and one-eighth inches in 

 diameter. We may appro.ximate the diffraction effect for this shape by 



