no 



BELL SYSTEM TECHNICAL JOURNAL 



500 1,000 

 FREQUENCY IN CYCLES PER SECOND 



5,000 10,000 20,000 



Fig. 12 — Pressure generated by a thermophone in a transmitter calibration chamber. 



Appendix II 



In the thermophone theory the walls of the chamber were treated as 

 being rigid. Actually the transmitter diaphragm presents a small but 

 finite admittance in shunt with the elastic admittance of the gas in the 

 chamber. The correction factor M due to this, is approximately 



M = 



1 



l+(2M!+2^.cos 



Vo 



Vo 



where 



7 = 1.4=^, 



assuming adiabatic conditions 



po = 10^ bars atmospheric pressure, 

 Vo = volume of thermophone chamber, 



V = volume displacement of diaphragm per bar, 



6 = phase angle of above displacement with respect to the pressure on 

 the diaphragm. 



At low frequencies cos 6 may be taken as nearly unity, and v can be 

 approximately computed as below 



