248 MEASUREMENTS 



Ve = total resistance of the strip, in ohms, 

 Ts = mean temperature of the strip, absolute degrees, 

 Ta = mean temperature of the gas in the enclosure, absolute degrees, 

 K — thermal conductivity of the gas, 

 p = density of the gas, in grams per cubic centimeter, 

 Cv = specific heat of the gas at constant volume, 

 Cp = specific heat of the gas at a constant pressure, 

 7 = Cp/Cv, 



po = average pressure of the enclosure, in dynes per square centi- 

 meter, 

 S = total area of one side of the thermophone foil, in square centi- 

 meters, 

 y = volume of the enclosure, in cubic centimeters, 

 CO = Itt/", and 



/ = frequency, in cycles per second. 

 The determination of the ratio e/p is carried out in the same manner as 

 the pistonphone. 



{c) Electrostatic Actuator ^. — The electrostatic actuator consists of an 

 auxiliary electrode in the form of a grill mounted in front of the micro- 

 phone diaphragm. Fig. II.IC. The actuator is perforated so that it does 

 not appreciably alter the impedance opposing the motion of the diaphragm. 

 A large steady polarizing voltage is applied to the grill and microphone 

 diaphragm. Then a sinusoidal voltage is applied, effectively, in series. 

 The alternating force in dynes per square centimeter of the grill, assuming 

 no tufting of the electrostatic lines, from equation 6.35 is 



p = ^ X 10- 11.3 



whereto = polarizing voltage, in volts, 



e = alternating voltage, in volts, and 



d = spacing between the actuator and the diaphragm, in centimeters. 

 The force developed by the actuator is independent of the frequency. 

 Therefore, it constitutes a simple system for obtaining the response of a 

 condenser microphone as a function of the frequency. If the absolute re- 

 sponse is desired this may be obtained by comparison with some known 

 standard. (Thermophone or pistonphone.) In the case of some actuator 

 structures the efi^ective area may be calculated from standard formulas 

 which correct for tufting. 



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



