Rayleigh wave attenuation

Rayleigh angle as usual. The value of (7.45) has the form of a ratio of impedances multiplied by a ratio of velocities, and it provides a means of relating Rayleigh wave attenuation (due to radiation into the fluid) to the density of the specimen. With the approximation of (7.45), the term in the large curly brackets in (7.42) becomes... 

Anyone who has successfully used a microscope to image properties to which it is sensitive will sooner or later find himself wanting to be able to measure those properties with the spatial resolution which that microscope affords. Since an acoustic microscope images the elastic properties of a specimen, it must be possible to use it to measure elastic properties both as a measurement technique in its own right and also in order to interpret quantitatively the contrast in images. It emerged from contrast theory that the form of V(z) could be calculated from the reflectance function of a specimen, and also that the periodicity and decay of oscillations in V(z) can be directly related to the velocity and attenuation of Rayleigh waves. Both of these observations can be inverted in order to deduce elastic properties from measured V(z). 

Fig. 8.5. Steps in the analysis of V(z) for fused quartz (Kushibiki and Chubachi 1985). (a) V(z) on a linear scale (b) V(z) filtered to remove short period ripple due to lens reverberations (c) V(z) for Teflon VL (d) Best value of V" after subtracting long period error in Vf (e) Fourier transform of (d) (f) Final Fourier transform from which the Rayleigh wave velocity and attenuation are found using eqns (8.36) and (8.37). 225 MHz, Ao = 6.6 m.

Fig. 8.8. Analysis of line-focus-beam V(z) data for 31 different materials and orientations, compared with calculated values, (a) Normalized measured period of the oscillations in V(z) versus calculated fluid-loaded Rayleigh wave velocity the curve is eqn (8.17). (b) Normalized measured attenuation from (8.19) versus calculated attenuation the line corresponds to perfect agreement (Kushibiki and Chubachi 1985).

Fig. 9.14. Buccal surface of human molar tooth. Rayleigh velocity and attenuation measured using a line-focus-beam lens at 225 MHz. The sketches indicate the approximate measurement angles at which the Rayleigh wave propagation was in the direction of the arrows... 

As mentioned above, the mass sensitivity of a 97 MHz SAW device exceeds that of the best APM sensitivity by a factor of two. However, because a 97 MHz Rayleigh wave propagating on ST-quartz suffers over 45 dB of Increased attenuation when water covers just 3... 

Surface waves (Rayleigh waves) occur when the beam enters the material at a shallow angle. They travel with little attenuation in the direction of the propagation, but their energy decreases rapidly as the wave penetrates below the surface. They are affected by variations in hardness, plated coatings, shot peening, and surface cracks, and are easily dampened by dirt or grease on the specimen. 

This method is applicable to materials which possess sufficiently low conductivity, for which the acoustoelecttic wave is not considerably quenched. Moreover, the nonideal coupling of the high-frequency amplitude into the Rayleigh wave amplitude must be taken into consideration, because in most cases the contacts are nonohmic, and scattering or attenuation of the wave occurs. These effects lead to underestimation of the mobility. 

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