Introduction

Abstract

Ultrasound can be thought of as analogous to ultraviolet light in that it characterizes that region of acoustical phenomena which is not accessible to human perception, because of the high frequencies involved. This term is by no means limited to sound waves travelling in gases and liquids but includes, in particular, the more complicated elastic waves in solids. Since the upper frequency limit of human hearing differs not only from one individual to another but also depends on age, the region of ultrasound cannot be divorced in a rigid manner from the range of audible sound. In general, however, we speak of ultrasound whenever the sound frequency exceeds 20 kHz.