An experimental study of a two-crystal acoustic interferometer



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An acoustic interferometer consisting of two flat 1 MHz quartz ciystals mounted in parallel configuration with a separation of approximately 1.2 cm was investigated for utilization in measurements of acoustic velocity and absorption. The separation of the crystals is held fixed and the response of the receiver crystal is measured as the frequency applied to the transmitting crystal is varied. From the frequency difference of the modes established in the cavity between the transducers, the ultrasonic velocity of the material between the transducers can be determined. From the width of any given mode, infonnation is obtained concerning the absorption coefficient of this medium. This interferometer closely resembles one described by F. Eggers (Acustica 19 (1967/68), 323). This design is unique in that it uses fixed crystal separation, extends the useful frequency range below 5 MHz for molecular relaxation time determinations and requires a small sample volume. It is found that Eggers' analysis must be modified to account for the frequency dependent phase shift of the waves upon reflection from the quartz crystals. This analysis accounts for the phase shift and affects an improvement in the accuracy of both velocity and absorption measurements.