ABSOLUTE CALIBRATION OF CONDENSER TR.ANS HITTERS 111 



1 ai^ yi 



AC ,j s 



2a.i2 h - 71 



2ai2V "^ // / '^ 3ai' h - 



AC C3 ^1 , 



2gi2 



where // = separation between diaphragm and back plate without 



polarizing voltage. 

 Ci = capacity between diaphragm and back plate without 



polarizing voltage. 

 C2 = above capacity in presence of polarizing voltage. 

 Cz = total transmitter capacity, with polarizing voltage. 

 £0 = polarizing voltage. 

 ei = transmitter e.m.f. per bar, uncorrected for yielding of 



diaphragm. 

 a I = diaphragm radius; 02 = back plate radius. 



For the 394-Type Transmitter, up to about 2,500 c.p.s., .1/ is nearly 

 0.92. Above that the correction decreases owing to decreasing cos 6, 

 and becomes negligible at 5,000 c.p.s. For still higher frequencies the 

 correction becomes negative but remains small due to the increasing 

 diaphragm impedance. 



Appendix III 



Schematically the membrane phone is shown in Fig. 6. D is the 

 diaphragm of the transmitter to be calibrated ; M, a stretched membrane 

 acoustically driven from the receiver R; G, a. perforated plate. Let 

 V = volume between D and M; yo = normal separation between G 

 and AI; Co = normal capacitance between G and M. 



Then, if yoll + KiS) • sin co/] represents the GAI separation when M 

 is driven by R, the resultant capacitance variation is: 



K(S) 

 z\c = sm cot- -—J — • I 



41lyo J 1 + 



and 



AC = sin CJt- -7-T — • I :; — ; — j^. . ... ■ ", 'dS 



AV = sin cot-yo I K{S)-dS, 

 the integration extending over the entire area of M. 



