Elastic Waves in Solids, Volume 2

3Generation and Detection

The propagation modes of elastic waves in an unbounded solid, on a free surface or at the interface between two media or in a plate, being known (Volume 1), it is necessary to examine the appropriate means of generating these waves and detecting them efficiently. The techniques implemented are very varied, insofar as they depend on practical constraints and technological possibilities.

If, for example, no mechanical contact with the sample is allowed, one solution could be to generate bulk waves or surface waves using a laser pulse, and to detect them, after propagation, using optical interferometry (Scruby and Drain 1990). At low frequency (f < 1 MHz) and when the acoustic impedance Z of the propagation medium is relatively small (Z < 10 MRayl), the coupling with the transducer can be achieved through the ambient air. Electromagnetic acoustic transducers (EMAT) are used in the industry to inspect metallic or ferromagnetic materials, by using transverse horizontal or guided waves (e.g. Thompson 1990).

However, the most common and efficient way is to exploit the piezoelectric effect, which couples mechanical and electrical quantities. The mechanism and the conditions of its existence in a crystal (absence of center of symmetry) have been described in section 1.4 of Volume 1. The electromechanical coupling coefficient, which characterizes the efficiency of the conversion, was introduced during the analysis of the propagation of bulk and surface waves. ...