A fixed path, variable frequency acoustic interferometer is described that is particularly suited for measuring ultrasonic velocities in liquids as a continuous function of temperature and pressure of the medium. The interferometer contains two clamped crystals: one, a low intensity ultrasonic source; and the other, a fixed, parallel reflector. The frequency of excitation, supplied from a frequency generator, is varied, and cyclic changes in input impedance are detected in an impedance bridge circuit. Crystal resonance is avoided. The interferometer, addition to the usual advantages of a fixed path system over a variable path one, operates at very low power levels; hence, it avoids problems occasioned by high intensity pressure waves. Frequency, the measured variable, is one that can be precisely measured with comparative ease.
The complete equivalent network for either the fixed or variable path interferometer is presented, from which the operation of the fixed path, variable frequency system is derived. The effect of crystal backing impedance is discussed, and the characteristics of the interferometer are shown to be those of a double crystal interferometer having infinite backing impedance.
The velocity of sound in water is measured between 25 and 53°C in the frequency range of 600 to 800 kc. At 25.0°C the measured velocity is 1496.8±0.3 meters/second, and the temperature coefficient is +2.7 meters/second °C.