for cyclically providing when actuated a transmit signal to a pulser 20 at a predetermined frequency typically at a frequency between 500 hertz and 10 kilohertz the pulser 20 upon receipt of a clock signal provides a high voltage transmit pulse to probe 12 for causing the transducer probe 12 to transmit an acoustic energy search signal into the rim of the wheel 14 the search signal typically is in the frequency range between 0 5 and 10 megahertz a portion of the search signal upon intercepting a defect or flaw in the wheel 14 is reflected back toward the probe 12 the amplitude of the defect responsive echo signal is indicative of the magnitude of the intercepted defect or flaw in the wheel 14 by adjusting the sensitivity of the defect detection unit for detecting defect responsive echo signals which exceed a predetermined minimum amplitude only wheel defects of a magnitude exceeding a predetermined minimum magnitude will cause a defect present signal to be manifest at the output of the defect detection unit the defect responsive echo signal received by the probe 12 is conducted to one input of a receiver amplifier circuit 22 where the defect responsive echo signal is.