Principles of Emar for Spectral Response

Abstract

An easy way to understand the EMAR principle is to consider the through-thickness resonances of bulk waves (thickness oscillations). Consider that a transducer is placed on the surface of a plate of thickness d and excites an ultrasonic plane wave of single cycle propagating in the thickness direction, whose wavelength and period are λ and T, respectively. The pulse wave undergoes the repeated reflections at both free surfaces and is received by the same transducer each time it reaches the incident surface. In the case of λ<<d, the received signals will be observed as shown in Fig. 4.1(a). The horizontal axis in Fig. 4.1 is the time measured from the excitation, and the vertical axis is the wave amplitude. T ₀ denotes the round-trip time through the thickness. The amplitude decays as the propagation path increases, depending on attenuation of the material.