Sound velocity is an essential part of this calculation. Different materials transmit sound waves at different speeds, and sound velocity in some materials--particularly plastics--varies with temperature. Thus, it's essential to calibrate an ultrasonic gage with the correct sound velocity for the material being tested, a process that requires consulting a reference standard of known thicknesses.
For any ultrasonic gaging application, the choice of gage and transducer depends on the test material, the thickness range covered and the accuracy required. Part geometry, temperature and any other special circumstances must be considered.