Sound waves, which are all around us, are simply organized mechanical vibrations traveling through a medium, which may be a solid, a liquid, or a gas. This applies to both the everyday sounds that we hear and the ultrasound used for flaw detection. Sound waves will travel through a given medium at a specific speed or velocity, in a predictable direction, and when they encounter a boundary with a different medium they will be reflected or transmitted according to simple rules. This is the principle of physics that underlies ultrasonic flaw detection. In short, ultrasonic waves will reflect from cracks or other discontinuities in a test piece, so by monitoring the pattern of echoes in a part a trained operator can identify and locate hidden internal flaws.