Ultrasonic testing uses pulsating waves that are imparted onto metals submersed in water. The waves penetrate the volume of the part and expose subsurface discontinuities throughout the base. Effective echo is measured as the waves are interrupted when they cross internal discontinuities. The back reflection is used to determine the integrity of the material being tested. Because of this characteristic, ultrasonic testing can also be used to determine wall thickness in parts not readily measurable by standard hard gauging.