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BELL SYSTEM TECHNICAL JOURNAL 



In particular the transmitter diaphragm and backplate may serve as 

 D2 and the fixed electrode. This, however, requires caution. The air 

 gap is so small (approximately 2.5 X 10~^ cm.) that unavoidable 

 variations in its value will in general cause appreciable variations in the 

 value of the electric driving force over dififerent parts of the diaphragm. 

 The non-uniformity of the air gap is due to mechanical imperfections 

 and to the electrostatic pull of the polarizing voltage. Furthermore, 

 in transmitters of the type here considered, the backplate diameter is 

 substantially smaller than that of the diaphragm, and hence the 

 compensating electric force is not effective in a peripheral portion of 

 the diaphragm. 



V k k ■. ^ ^ ^ ^ k k k k k t k k k k ^ ^ k ■.». u k ■■■.■. ^ ^1 /T?^ "I 



Fig. 5 — Electrodynamic compensation method. 



The electric force in this case is provided by inserting between the 

 diaphragm and the backplate a steady potential difference, Fo, and a 

 much smaller alternating potential difference, Vi sin wt, in series. One 

 of the resultant force components is aV^Vi sin o^t which has the same 

 frequency as the sound source (e.g. a thermophone). The amplitude 

 and phase of the electric force are adjusted until it balances the 

 acoustic pressure on the diaphragm. This gives the value of the 

 acoustic pressure, provided a is known. The compensating electric 

 force is then removed, and the output of the transmitter due to the 

 acoustic pressure is measured. Thus the pressure calibration is 

 obtained. The value of a is given by a measurement of the value of 

 Fo required to balance a known static gas pressure established at the 



