Increasing the sensitivity of the scanning acoustic microscope to anisotropy
The response of the scanning acoustic microscope to anisotropic materials is theoretically investigated. For this purpose, the reflection coefficient of plane acoustic waves incident on a liquid-anisotropic-solid interface is calculated. The reflection coefficient depends, in general, on polar and azimuthal angles of incidence. For the acoustic microscope case, a mean reflectance function can be defined which depends only on the polar angle, because there is a circular symmetry. With this mean reflectance function it is possible to explore the effects of changing the lens parameters such as the acoustic field at the back side of the lens. It is found that the response of the scanning acoustic microscope can depend heavily on the orientation of the solid material under investigation, provided that a suitable lens insonification is utilized. The amplitude of the acoustic microscope signal is influenced by the orientation of the material, because there is an interference between the acoustic waves reflected from the material surface at different azimuthal angles. This interference is revealed as a minimum in the mean reflectance function. It is shown by computer simulation that sensitivity to orientation can be increased by use of a ring-shaped transducer in the near field of the acoustic lens. With such lenses, it may be possible to determine the orientation of crystallites in a material.