High frequency sound waves can often be successfully transmitted into and out of moving materials without direct contact, through the use of a water bath or water stream as a coupling medium. Measurements can also be performed within closed containers by coupling sound energy through the wall. Because sound waves penetrate through the test specimen, material properties are measured in bulk rather than just on the surface. Additionally, varying degrees of nonhomogeneity may have an effect on sound velocity. Sound energy is absorbed or attenuated at different rates in different materials, governed in a complex fashion by interactive effects of density, hardness, viscosity, and molecular structure.
Attenuation normally increases with frequency in a given material.
Changes in grain structure, fiber orientation, porosity, particle concentration, and other microstructural variations can affect the amplitude, direction, and frequency content of scattered signals. Scatter effects can also be monitored indirectly by looking at changes in the amplitude of a backwall echo or a through-transmission signal. Thus, analysis of changes in the remaining frequency content of a selected broadband pulse that has passed through the test material can track the combined effects of attenuation and scattering as described above. In some applications ultrasonic data such as velocity can be directly used to calculate properties such as elastic modulus. In other cases, ultrasonic testing is a comparative technique, where in order to establish a test protocol in a given application it will be necessary to experimentally evaluate reference standards representing the range of material conditions being quantified.