Abstract: Apparatus for detecting an abnormality such as a discontinuity in a predetermined region of a material by directing an elastic mechanical wave, such as an ultrasonic or other acoustic wave, in a series of pulses to portions of the region and providing response to the wave pulses reflected therefrom. The amplitudes of pulses reflected from a normal portion of the region may approximate the amplitudes of pulses reflected from a portion where an abnormality is present and thus make it difficult to distinguish between such portions.The directed wave is varied, as by rotating a transducer having an irregular cross-sectional radiation pattern, to provide randomlike variations in the shapes of successive pulses where reflected from a normal portion, differing detectably from any variations caused in the pulses where reflected from an abnormal portion.