MEASUREMENTS AT THE SPEED OF ULTRASOUND: FRIENDLY REGARDS FROM FERMAT by

The word “bent” has a special meaning to readers of THE BENT. In construction or architecture, a bent is a structural element used in post and beam construction. Its more common meaning brings to mind a shape like a rod or a line that seems headed one way but then takes a sudden turn. A common example is a stick held in water at an angle, partly immersed, partly in air. It appears bent. The explanation is familiar. The speed of light is about 33% faster in air than in water. Therefore the light rays bend (refract) when they strike the interface obliquely. This article explores how this simple concept underlies ultrasonic devices used in two different fields: NDT & PCI (nondestructive testing and process control instrumentation). Conversely, we’ll also try to show that devices that look pretty simple can lead one to fundamental design principles, or at least illustrate such principles in action. The device we’ll look at is an acoustic wedge, and its purpose is to bend ultrasonic rays. But first, let’s return to our stick in water.