The Pierce Acoustic Interferometer as an Instrument for the Determination of Velocity and Absorption

Sputtered quartz crystals of known natural frequency were used as sources of high frequency sound in a Pierce acoustic interferometer. The interfering sound waves react on the crystal driver in such a way as to change periodically the plate current, $I_p$, when the mirror is displaced a half wave-length or more. The galvanometerdeflection, mirror-displacement curves show that multiple reflections take place at the movable mirror and at the crystal surface. The general agreement of these curves and those which represent the resultant pressure amplitude in the gas at the crystal surface as ordinates and the mirror displacement as abscissa shows that the changes in galvanometer deflection are a direct function of the changes in pressure amplitude. Check runs made with a torsion vane detector indicate that this function expresses a proportionality between the deflection change and the square of the change in the pressure amplitude, provided the latter change remains small.