Coherent Detection Technique for Variable-Path-Length Measurements of Ultrasonic Pulses

Abstract

A technique for the accurate measurement of ultrasonic velocity and attenuation in fluids is presented. The additional delay and decrease in amplitude of an ultrasonic pulse, resulting from an increase in the length of the transmission path, are precisely measured by phase comparison of the received ultrasonic signal with a continuous coherent reference signal at the carrier frequency. Phase-sensitive detection in a balanced mixer provides a phase-detector output, which is linear, sensitive to small delay changes, and has a high signal-to-noise ratio. The technique is well suited for measurements in highly attenuating fluids. Ultrasonic propagation near the critical point of xenon was studied using this technique. Attenuations in excess of 400 dB/cm were measured at 1 MHz.