Elastic Wave Propagation and Associated Phenomena

This chapter is intended to acquaint the reader with some of the fundamental principles involved in the various applications of ultrasonic energy. In many respects, ultrasound is similar to light and other forms of electromagnetic radiation in that it is a wave motion and obeys a general wave equation. Ultrasound is a form of mechanical energy, a vibration or wave-field, that propagates through solids, liquids, and gases. As a wave propagates through a medium, its amplitude decreases, or attenuates. There are several causes of this attenuation, such as spreading of the wave front, conversion of the acoustical energy to heat, and scattering from irregular surfaces. An ultrasonic wave may be considered to consist of an infinite number of oscillating masses, or elements, connected by means of elastic springs so that each element is influenced by the motion of its nearest neighbor. In acoustics, various phenomena cause a lag between an applied strain and the resulting stress, or vice versa.