Page 340 - Microsensors, MEMS and Smart Devices - Gardner Varadhan and Awadelkarim

320 SURFACE ACOUSTIC WAVES IN SOLIDS

Slobodnik 1976). This chapter on the basic understanding of wave theory, together with
the next chapter on measurement theory, should provide all readers with the neces-
sary background to understand the application of interdigital transducer (IDT) microsen-
sors and microelectromechanical system (MEMS) devices presented later in Chapters 13
and 14.

10.2 ACOUSTIC WAVE PROPAGATION

The most general type of acoustic wave is the plane wave that propagates in an infinite
homogeneous medium. As briefly summarised at the end of Chapter 9 for those readers
omitting that chapter, there are two types of plane waves, longitudinal and shear waves,
depending on the polarisation and direction of propagation of the vibrating atoms within
the medium (Auld 1973a). Figure 10.1 shows the particle displacement profiles for these
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two types of plane waves . For longitudinal waves, the particles vibrate in the propaga-
tion direction (y-direction in Figure 10.1 (a)), whereas for shear waves, they vibrate in a
plane normal to the direction of propagation, that is, the x- and z-directions as seen in
Figure 10.1(b) and (c).
When boundary restrictions are placed on the propagation medium, it is no longer an
infinite medium, and the nature of the waves changes. Different types of acoustic waves
may be supported within a bounded medium, as the equations given below demonstrate.
Surface Acoustic Waves (SAWs) are of great interest here; in these waves, the traveling

Figure 10.1 Particle displacement profiles for (a) longitudinal, and (b,c) shear uniform plane
waves. Particle propagation is in the y-direction

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Also see Figure 9.10 in Chapter 9 on the introduction to SAW devices.