Ultrasound is sound above 20 000 hertz, too high to hear. A probe sends a pulse into the metal; wherever it meets a boundary, such as a hidden crack, part of it bounces back. The deeper the flaw, the longer the echo takes to return, since depth equals the speed in the metal times the echo time divided by two. The trace below shows the echoes arriving. Move the flaw to make its echo line up with the target gate.