CALIBRATION OF MICROPHONES 247 



This method is very useful for calibrating a microphone at the low fre- 

 quencies. The upper frequency limit is governed by the permissible speed 

 of the mechanical system which is approximately 200 cycles. 



Under test the output of the microphone is fed to an amplifier and out- 

 put meter. For a particular value of generated pressure the output is 

 noted. Then, the pistonphone is disconnected and a voltage of the same 

 frequency as that generated by the pistonphone is inserted in series with 

 the microphone and adjusted to give the same output. The response {elp) 

 at this frequency is the ratio of this voltage to the applied pressure. 



(b) Thermophone^'^''' . — The thermophone consists of one or more 

 strips of thin gold leaf mounted upon terminal blocks, Fig. 11.15. In the 

 usual method the thermophone strip carries a known steady current upon 

 which a smaller sinusoidal current is superimposed. In this case, the vari- 

 ation of the pressure in the chamber occurs primarily at the frequency of 

 the alternating current. The cavity of the thermophone is usually filled 

 with hydrogen. The wavelength in hydrogen is considerably longer than 

 in air and, as a consequence, the standing waves are shifted to a higher 

 frequency beyond the useful response range. 



The peak alternating pressure developed in the cavity is given by 



SGSkirE 11 ^ 



P = ^~r79 11-2 



( 4KS^V ( 4.V 4KSa \KS''\ 



^CVA> 



TsT - 1 



(t - 1)T, 



7^0 



i- 



IK 



C = total thermal capacity of the strip, product of the mass in grams 



and the specific heat, 

 io = steady current, in amperes, 

 i = peak value of the alternating component, in amperes, 



5 Arnold and Crandall, Phys. Rev., Vol. 10, No. 1, p. 22, 1917. 



« Wente, E. C, Phys. Rev., Vol. 19, No. 4, p. 333, 1922. 



^ Ballantine, S., Jour. Acous. Soc. Amer., Vol. Ill, No. 3, p. 319, 1932. 



