Abstract: This invention relates to contactless proximity sensing transducers and more particularly concerns a single crystal ultrasonic interferometer comprised of a piezoelectric crystal attached to a Pyrex plate for acoustic impedance matching purposes. When driven at the crystal's resonant frequency, the electrical impedance of the crystal becomes sensitive to the input acoustical impedance of the interferometric path length between the crystal and the target object. By monitoring the crystal impedance at a fixed frequency as target position is varied, the sensor is capable of determining the incremental displacement of the moving target. By monitoring the crystal impedance at a fixed target position as the driving frequency is varied, the sensor is capable of determining the absolute proximity of the target. The geometry of the piezoelectric crystal and the Pyrex plate are used to affect the resolution and frequency response of the sensor.