Abstract: To detect the presence of certain types of hidden flaws within a material having an exposed surface, the surface is tapped at one location to produce sonic vibrations within the material. At another location, an acoustic receiving transducer is coupled to the surface of the material being tested. A characteristic of the electrical signals produced by the transducer which is indicative of the type of flaw to be detected is utilized to denote the presence of a flaw. A mobile foorm of apparatus which detects flaws while traversing the surface of the material and which indicates the location as well as the existence of the flaws is described.