The basic principle is that of “ECHO” reflection of ultrasound by voids. High frequency electrical signals are converted as mechanical vibrations in a peizo-electric crystal. These vibrations form a wave front and are coupled to parts under inspection with suitable medium such as oil, grease or water. These vibrations propagate through material in longitudinal, shear or surface modes and are reflected by any metal –air interfaces that are oriented approximately normal to incident sound wave. The reflected waves from such interfaces or flaws are directed back to the same transducer and are converted as electrical signals to the presented as a pip or vertical deflection in cathode ray tube screen (CRT) of an ultrasonic flaw detector

Straight beam ultrasonic inspection, where sound beam enters vertically from the surface of inspection is specified to detect lamination in plates and internal discontinuities in forgings and castings. Angle beam inspection, where sound enters inside the material at predetermined angles to the surface, is specified for welds, rings and pipes to detect cracks and inclined flaws