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The term ultrasonics applies to sound waves that vibrate at a frequency higher than the frequency that can be heard by the human ear (or higher than about 20,000 hertz).

Sound is transmitted from one place to another by means of waves. The character of any wave can be described by identifying two related properties: its wavelength (indicated by the Greek letter lambda, λ) or its frequency (f). The unit used to measure the frequency of any wave is the hertz (abbreviation: Hz). One hertz is defined as the passage of a single wave per second.

Ultrasonics, then, deals with sound waves that pass a given point at least 20,000 times per second. Since ultrasonic waves vibrate very rapidly, additional units also are used to indicate their...