It is a volumetric NDE method in which beams of high frequency sound waves that are introduced into the material being tested are used to detect volumetric flow. The sound waves travel through the materials with some attenuation of energy and are reflected at interfaces. The reflected beam is detected and analyzed to define the presence and location of flaws. (Cracks, laminations, shrinkage, cavities, bursts, flakes, pores, bonding faults and other discontinuities that can act as metal-gas interfaces can be easily detected.)