UT utilizes mechanical waves (vibrations) that travel well through many materials, especially metals. The technique has proven to be extremely useful in finding structure-critical material irregularities. Sound waves travel at rates that are determined by the material's physical characteristics of elasticity, density, and homogeneity. UT is used in the testing of materials such as fine-grained metals (aluminum), and for the detection of inter-laminar separations and regions that have been improperly processed or damaged in layered composite structures. Some advantages of UT include its high sensitivity to defects that are most critical to structural integrity (e.g. cracks). It is characterized by its ease of use, portability of the testing equipment, and applicability to the testing of a wide range of materials.