Echo and Shadow of an Obstacle in the Sound Field

Abstract

A given flaw in a test object, e.g. a cavity in a casting, represents an obstacle to the sound wave. Information on this flaw can only be obtained either via the reflected wave, i.e. its echo when using the echo method, or by its shadow when applying the method of sound intensity measurement. Information obtained from the echo and the shadow on the basis of purely geometric-optical concepts can be obtained only in the case of very large and regularly shaped flaws: in the case of a flat, smooth transverse crack in a roll the incident wave is reflected as on a mirror, resulting in a total shadow zone behind the crack. In most instances however natural defects are so small, that diffraction phenomena determine both the echo and the shadow. Therefore defects are classified according to their dimensions transverse to the beam, as large or small defects. The designation small of course constitutes no evaluation of the seriousness of the defect in respect of the usefulness of the test object concerned.