2. Principle of the measurement method

Applying an electrical voltage to the piezoelectric element in a transducer causes the piezoelectric material to oscillate, creating sound waves that travel from the transducer, through the liquid or coupling medium and into the screw. This type of ultrasonic wave travels the length of a screw. The acoustic velocity of a material represents the speed at which the wave travels through it. All materials have a representative, or theoretical, acoustic velocity, but the actual acoustic velocity can vary from sample to sample of the same material, and even throughout the material of a particular sample.