. Elements of acoustical engineering. Electro-acoustics; Sound. 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.
. Elements of acoustical engineering. Electro-acoustics; Sound. 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. 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 ( for air). ^/^^ = ,20°C, Aw = areaof metallic walls, in square centimeters, K = thermal conductivity of the enclosed gas ( 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, Please note that these images are extracted from scanned page images that may have been digitally enhanced for readability - coloration and appearance of these illustrations may not perfec
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