Ultrasonic wave propagation is influenced by the microstructure of the material through which it propagates. The velocity of the ultrasonic waves is influenced by the elastic moduli and the density of the material, which in turn are mainly governed by the amount of various phases present and the damage in the material. Ultrasonic attenuation, which is the sum of the absorption and the scattering, is mainly dependent upon the damping capacity and scattering from the grain boundary in the material. However, to fully characterize the attenuation required knowledge of a large number of thermo-physical parameters that in practice are hard to quantify.
Relative measurements such as the change of attenuation and simple qualitative tests are easier to make than absolute measure. Relative attenuation measurements can be made by examining the exponential decay of multiple back surface reflections. However, significant variations in microstructural characteristics and mechanical properties often produce only a relatively small change in wave velocity and attenuation.
Absolute measurements of attenuation are very difficult to obtain because the echo amplitude depends on factors in addition to amplitude. The most common method used to get quantitative results is to use an ultrasonic source and detector transducer separated by a known distance. By varying the separation distance, the attenuation can be measured from the changes in the amplitude. To get accurate results, the influence of coupling conditions must be carefully addressed. To overcome the problems related to conventional ultrasonic attenuation measurements, ultrasonic spectral parameters for frequency-dependent attenuation measurements, which are independent from coupling conditions are also used. For example, the ratio of the amplitudes of higher frequency peak to the lower frequency peak, has been used for microstructural characterization of some materials.