Page 341 - Microsensors, MEMS and Smart Devices - Gardner Varadhan and Awadelkarim

INTRODUCTION TO ACOUSTICS 321

y-polarized

x-polarized

z-polarized

z-polarized
x-polarized
y-polarized
x-propa

Figure 10.2 Acoustic shear waves in a cubic crystal medium

wave is guided along the surface with its amplitude decaying exponentially away from the
surface into the medium. Surface waves were introduced in the last chapter and include
the Love wave mode, which is important for one class of IDT microsensor.

10.3 ACOUSTIC WAVE PROPAGATION
REPRESENTATION

Before a more detailed analysis of the propagation of uniform plane waves in piezoelectric
materials in the following sections, a pictorial representation of the concept of shear wave
propagation is presented. Figure 10.2 illustrates shear wave propagation in an arbitrary
cubic crystal medium. An acoustic wave can be described in terms of both its propagation
and polarisation directions. With reference to the X, Y, Z (x, y, z) coordinate system,
propagating SAWs are associated with a corresponding polarisation, as illustrated in the
figure.

10.4 INTRODUCTION TO ACOUSTICS

10.4.1 Particle Displacement and Strain

As stated earlier, acoustics is the study of the time-varying deformations or vibrations
within a given material medium. In a solid, an acoustic wave is the result of a deformation
of the material. The deformation occurs when atoms within the material move from their
equilibrium positions, resulting in internal restoring forces that return the material back
to equilibrium (Auld 1973a). If we assume that the deformation is time-variant, then