How does ultrasound detect cracks or imperfections in metal? Is this to do with density or is it just reflecting off surfaces?
Like all waves, ultrasound reflects whenever it passes from one material to another and experiences a change in speed (or more accurately, a change in impedance). Any inhomogeneity in a metal is likely to change the speed of sound in that metal and will cause some amount of sound reflection. With the proper instruments emitting sound and detecting the reflected sound, it's possible to image the imperfections. The same technique is used in medical ultrasound to image organs or fetuses, and even to image the insides of the earth.
Why do we say that a bad deal will only “Rob Peter to pay Paul”?