Endosonography: Physical Basics and Instrumentation

Abstract

“Ultrasound” is the name given to a class of mechanical pressure waves that can be propagated to liquids, solids, and, to some extent, gases [1], Mechanical waves have frequencies which range from little more than 0 Hz up to several hundred million. For convenience, this vast range may be represented in the form of an acoustic spectrum, which is analogous to the more familiar spectrum of electromagnetic radiation. A typical frequency spectrum of mechanical waves is represented in Fig. 2.1, where a logarithmic scale has been chosen so that each tenfold increase in frequency is represented by an equal distance. The spectrum can be seen to consist of three broad regions, which overlap at their boundaries. The central region encompasses the audible spectrum from about 20–30 Hz up to about 16 Hz, these frequencies being the approximate lower and upper frequency limits of the average human ear. The region below about 20 Hz is designated infrasound, while frequencies greater than about 16 kHz are called ultrasound (US).