112 BELL SYSTEM TECHNICAL JOURNAL 



Taking K{S) < < 1, but without restrictions on the variation of 

 K{S) over the surface of M, 



AV = 4n3/o^-ACo. 



Hence the transmitter sensitivity is given by 



62 e^VEo . ., 



R = — = 3^ volts/bar. 



The above presupposes: (1) V/S<<X, ^jS<<X•, (2) acoustic 

 admittance of D is very small compared with that of V; (3) adiabatic 

 compression. If necessary, corrections for deviations from (2) can 

 be made in accordance with Appendix II. The correction for (3) is 

 found to reduce the pressure in the ratio 



R' = R ^ 



l + (^_l),tanh^a' 



where 



13a 



;5 = (. + i)V#. 



when C = specific heat at instant pressure, 

 K = thermal conductivity of the gas, 

 p = density. 



The upper frequency limit imposed by condition (1) can be raised by 

 filling F with hydrogen. For the No. 394-Type Transmitter, and with 

 R a No. 555-W Western Electric Receiver, an air-gap yo = 0.075 cm. 

 corresponds to easily measurable values of ei and e^. M was a 0.001 

 inch duralumin diaphragm, stretched to 5,000 c.p.s. resonance fre- 

 quency. It was found that the upper frequency limit of the method is 

 determined by M breaking up when vibrating in one of its higher 

 natural modes. This tends to produce a non-uniform pressure on D, 

 and the above condition must be met much more perfectly than in the 

 thermophone case. 



Appendix IV 



The particular electrostatic calibration described below, employs a 

 separate driving electrode and a sinusoidal driving voltage which 

 produces a sinusoidal driving force of double frequency. The latter 

 has the advantage of adding frequency selectivity to shielding as the 



