The Acoustic Resonator Interferometer: II. Ultrasonic Velocity and Absorption in Gases

The derivation of the equivalent electric network of the acoustic resonator interferometer in Part I of this paper has made it possible to develop the theory for the current in a simple resonant circuit in which the electrodes of the piezoelectric plate of the interferometer are connected to the terminals of the variable condenser of the circuit. The special case of this theory in which the circuit is excited at a constant frequency determined by the crevasse frequency of the resonator plate in its given situation with respect to electrodes and associated circuit, when the acoustic path in the interferometer is detuned and the resonant circuit is tuned so that its resonant maximum occurs at the same frequency, takes an especially simple form and leads to a direct procedure for determination of ultrasonic velocity and absorption in a gas in terms only of current in the resonant circuit and path-length in the interferometer, all circuit and interferometer constants dropping out. The values of current as a function of path-length obtained experimentally are in complete accord with the theory and data for ultrasonic absorption in air and in C${\mathrm{O}}_2$ so far obtained are in agreement with the meager data available by other methods. The role of the coefficient of reflection at the fluid-reflector surface is discussed.