Section 2: Physics of Ultrasound

Scattering:
Grain Size and Wave Frequency
The relative impact of scattering source of a material depends upon their
grain sizes in comparison with the Ultrasonic sound wave length. As the
scattering size approaches that of a wavelength, scattering by the grain is a
concern. The effects from such scattering could be compensated with the use
of increasing wavelength ultrasound at the cost of decreasing sensitivity and
resolution to detection of discontinuities.
Other effect are anisotropic columnar grain with different elastic behavior at
different grain direction. In this case the internal incident wave front becomes
distorted and often appear to change direction (propagate better in certain
preferred direction) in respond to material anisotropy.