420 



BELL SYSTEM TECHNICAL JOURNAL 



This cjuantity when plotted against frequency constitutes the contour 

 pressure caHbration. The normal incidence field calibration is found 

 by always adding at any frequency the larger ordinate of the curves 

 representing the diffraction of a sphere and of a flat plate. For con- 

 venience these effects shown already in Fig. 6 are given again in Fig. 10 

 which also compares the theoretical with the experimental response. 

 We observe that the theoretical field calibration is in good agreement 

 with the experimental response. 



The meaning of most constants used in evaluating (5) is evident. 

 Some are easily calculated, while others have to be found by measure- 

 ment. The resistance of the silk is found by allowing air of a certain 



500 1000 



FREQUENCY IN CYCLES PER SECOND 



5000 10,000 20,000 



Fig. 10 — Field response of 630-A microphone without screen 

 for sound incident normally to diaphragm. 



volume velocity to flow through it and by measuring at the same time 

 the pressure drop across the resistance.^ The mass of the silk is found 

 by impedance measurements.^ To find the equivalent mass of the coil 

 slot is somew^hat difficult since it consists of the mass of the slot plus a 

 certain mass under the dome and under the outer annulus. If the 

 separations between diaphragm and magnet structure are large the 

 problem becomes much simpler since only the mass in the coil slot 

 needs to be considered. The velocity in the slot varies along its width 

 and for any point is given by 



^. ^ \dp Z{Z-h) 

 fx dx 2 



(6) 



where Z is the distance from the side wall of the slot to the point in 



