Ultrasonic testing (UT) utilizes high frequency sound to determine metal thickness. Based on the same principle as SONAR, a sound beam is introduced into a part and the thickness is determined using the time required for the sound beam to travel to the back of the surface and return. This is done by converting the "time of flight" of the sound to a corresponding computed thickness.
UT can also be used to inspect items for "soundness", determining the presence of flaws by the continuity of the sound path. A discontinuity (such as a crack) creates a disruption in the sound beam that is seen as a "blip" on the "radar" screen.