Ultrasonic thickness meters use ultra sound for measuring - mechanical vibrating of material environment similar to audible sound. But unlike the audible sound is the frequency of ultrasound beyond audibility. The thickness measuring by ultrasound is enabled only due to the fact that ultrasound easily spreads through solid materials.
Ultrasoud is created in the probe, which is then applied on the surface of the tested material. Ultrasound is not sent out to the material permanently but in short impulses. The impulse spreads through the material, echoes from the opposite surface, or alternatively the internal flaw, and returns back to the probe, which then transfers the ultrasound into electric signal and which is then further processed in the device.