UT uses complex electronic equipment. Any material which transmits mechanical vibrations can be tested. UT detects both linear and non-linear flaws and permits three dimensional interpretations. Evaluation is often difficult. The UT instrument converts electrical pulses into mechanical vibrations or waves. These waves travel across the tested specimen and reflect from flaws because of their different acoustic nature. The returning reflected waves are reconverted to electric energy and displayed as signals on a cathode ray tube (CRT). The position and size of these signals correspond to the position and size of the flaws.