246 



MEASUREMENTS 



microphone. The ratio ejp is usually expressed in decibels with respect 

 to some arbitrary reference level. The pressure upon the diaphragm may 

 be generated by a pistonphone, thermophone or an electrostatic actuator. 

 (a) 'Pistonphone'^''^. — A schematic arrangement of a pistonphone for 

 use in calibrating a pressure type microphone having a diaphragm of high 

 acoustic impedance is shown in Fig. \\.\A. The small piston is driven 



CONDENSER 

 MICROPHONE 



diaphragm] 



PISTONPHONE 



THERMOPHONE 



ELECTROSTATIC ACTUATOR 



Fig. 11.1. Apparatus for obtaining the pressure frequency characteristic of a condenser type 

 microphone. The pistonphone and thermophone may be used for other types of pressure 

 microphones. 



by a crank. The pressure generated at the diaphragm, assuming all of 

 the walls of the enclosure to be rigid, is 



1-1/2 



rApA (7 - \)A^ ^ 1 / (t - \)A: \\ 



where p = peak pressure, in dynes per square centimeter, 

 V^ = volume of the enclosure, in cubic centimeters, 

 A = area of the piston, in square centimeters, 

 r = radius of the crank, in centimeters, 

 p^ = atmospheric pressure, in dynes per square centimeter, 

 7 = ratioof specific heats (1.4 for air). 



11.1 



^/^^ = 3.9V7forair,20°C, 



Aw = areaof metallic walls, in square centimeters, 



K = thermal conductivity of the enclosed gas (6.2 X lO"^ for air) 



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

 Cp = specific heat of the gas at constant pressure (.24 for air), 



CO = 271/, and 



/ = frequency, in cycles per second. 



^ Olson and Massa, "Applied Acoustics," 2nd. Ed., p. 267, P. Blakiston's Son 

 and Co., Philadelphia, 1939. 



^ A dynamically driven pistonphone has been described by Glover and Baumz- 

 weiger. Jour. Acotis. Soc. Amer.^ Vol. 10, No. 3, p. 200, 1939. 



