Waves propagation

Wave phenomena are used to evaluate material properties nondestructively as well as to locate and measure defects in critical structures. In fact, physical acoustics is directly related to ultrasound since the ultrasonic waves are employed to determine the properties of materials. The objective of this chapter is to present a brief overview of the physics involved in the propagation of the sound through a material. We start with a description of the physics of the acoustic wave considering the governing equations and the parameters that characterize the means in order to obtain the wave equation in the time domain, which results in the Helmholtz equation in the frequency domain. Successively, various phenomena of wave propagation are described. There is much information available in the acoustic wave that is transmitted and received through a material. In particular, the reflection of elastic waves at a free boundary and reflection and refraction at an interface can be used in order to detect an anomaly in the material tested