Ultrasonic Waves in Free Space

Abstract

Ultrasonic testing of materials utilises mechanical waves in contrast, for instance, to X-ray techniques which use electromagnetic waves. Any mechanical wave is composed of oscillations of discrete particles of material. The motion carried out by a small mass attached to a spring as shown in Fig. 1.1 if pulled down once and released, is called an oscillation. Left to itself, the mass oscillates about the equilibrium position. The nature of this oscillation is of particular importance inasmuch as it is sinusoidal, the path recorded as a function of time being a sine curve. It is obtained only if the driving force, in this case supplied by the spring, increases proportionately to the displacement. It is then also referred to as an elastic oscillation. Furthermore, one can imagine the body to consist of individual particles kept in position by elastic forces. Very much simplified, the model of an elastic body can be visualised as shown in Fig. 1.2, but three-dimensionally. Provided such a body is not stressed by compression or tension beyond its elastic limit, it behaves like this spring model. In it, the particles can perform elastic oscillations. How then does a wave arise from an oscillation?