Abstract: In an ultrasonic pulse-echo test circuit a probe means is coupled via a liquid filled coupling path to a workpiece which undergoes motion relative to the probe means. For each search signal transmitted by the probe means the transit time of such signal is measured from the time of transmission by the probe means to the receipt of the workpiece rear surface responsive echo signal. A computer coupled to receive the transit time values is programmed to classify transit time changes as fast occurring changes, caused by changes in coupling path length, or as slow occurring changes, caused by workpiece thickness changes. A defect gate forming a part of the test circuit is controlled by adjusting its opening responsive to a change in coupling path length and its closing responsive to a workpiece thickness change.