Above the auditory threshold
In the lab, a coupling medium (such as adhesive, gel, water, or oil) is first applied to the surface of the material; this will ensure that the waves are transmitted from the transducer to the test object. A moveable testing probe is then moved across the surface of the object. Imperfections in castings most frequently include gaps, pores, cracks, or other kinds of air inclusions; these then reflect the ultrasonic waves and are bounced back to the test probe, indicating the presence of a disturbance. (In many cases, the sender and receiver units are both integrated directly into the probe, although devices with separate sender and receiver components are possible.) It is also possible for a significant part of the signal to be lost; this can have several causes, but can also be an indication for testing errors.