The term Ultrasonics or supersonics refers to sound vibrations i.e. variations of density in elastic media e.g., air whose frequencies are beyond the auditory limit, i.e., above approx. 20,000 cycles/sec. Such high frequency elastic vibrations are produced in various ways, based on different physical principles.

An obvious method, in the first place, is to extend the old acoustic principle of sound generation by means of pipes to the ultrasonic range. This can be done by means of the Galton pipe (Fig.1) This is a pipe in which the position of the gap and lip can be varied by micrometer adjustment. It is blown by compressed air and enables ultra-sound with frequencies up to 30,000 cycles/sec. to be produced. Higher frequencies can be attained by making use of other phenomena, namely, magnetostriction and the piezoelectric effect. Magnetostriction is the change in the dimensions of a ferromagnetic material when it is placed in a magnetic field. If the latter is produced by an alternating current, the material will undergo vibrations.

Fig.2 is a diagrammatic representation of a magnetostriction ultrasonic generator. It consists of 0.1 – 0.3 mm (0.004 – 0.012 in) thick nickel plates, insulated from one another, whereby the eddy current losses are reduced. The general construction is rather like that of a transforme. The arrangement of the windings, as indicated in the diagram, causes the magnetic field to form a closed circuit in the stack of plates. Utilizing this principle, ultrasonic waves with frequencies of up to 200,000 cycles/sec. can be produced. The sound is radiated e.g., in air or water in two directions (Fig.2). If radiation is required in one direction only, one face must be provided with a foam rubber cushion, which acts as a screen impervious to sound.

Even higher frequencies can be attained by means of the piezoelectric effect. A circular quartz plate cut from a hexagonal quartz crystal in that manner shown in Fig.3 is so gripped at its edge (Fig.4) that thin metal foils on the two circular faces of the plate impart a high-frequency charge to it and that it is nevertheless able to vibrate freely. The high-frequency thickness vibrations caused by the piezoelectric effect are transmitted to the air in a sound box and thence via a diaphragm to an adjacent medium e.g., water.

Ultrasonic vibrations are used in many technical applications, including non-destructive testing of materials, degasification of liquids, echo-sounding, and also in therapeutic medicine. Certain nocturnal animals, such as bats, make use of ultrasonic vibrations to guide their movements in the dark on the radar principle.