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BELL SYSTEM TECHNICAL JOURNAL 



along the diaphragm the effective pressure is reduced at the higher 

 frequencies due to difference in phase. Hence, if the direction of 

 approach of the sound wave is parallel to the plane of the diaphragm, 

 the departure from flatness is materially reduced. This is brought 

 out quite clearly by the field calibration for sound approaching from 

 a direction parallel to the diaphragm, Fig. 6.'-* 



The discrepancy between the pressure and field calibrations of the 

 condenser microphone involves two important assumptions, namely, 

 a plane sound wave and no reflection from walls or surrounding objects. 

 When the microphone is used in a studio much of the sound reaches 

 the diaphragm by way of reflection from the walls of the room. The 

 requirement of no reflection is therefore not met and the influence of 

 the acoustic properties of the reflecting surfaces is added to the char- 

 acteristics of the microphone. The effect of the diffusion of the 



-2P 



1000 

 FREQUENCY IN CYCLES PER SECOND 



Fig. 6 — Field calibration of the 394-type condenser microphone for a direction of 

 approach of sound parallel to the diaphragm. 



sound field and the tendency for most materials to be more absorbent 

 for sounds of high frequency appears to cause the response under 

 studio conditions to be more nearly like that obtained when the sound 

 approaches in a direction parallel to the diaphragm and make the 

 departures from the pressure calibration less marked than the field 

 calibration for a direction normal to the diaphragm would indicate. 

 This perhaps accounts in part at least for the instances in which a 

 corrective network designed to compensate for the field calibration 

 normal to the diaphragm failed to effect a material improvement in 

 quality. 



The acoustic conditions under which a microphone is used cover a 

 wide range. It would therefore be difficult if not impossible to adopt 

 a set of conditions for use in connection with a field calibration of the 

 condenser microphone, which would be known to be representative 

 of those encountered in practice. The pressure method of calibration 



