Sound velocity
Ultrasonic detection involves measuring both the velocity of sound in a solid and the distance it travels. Sound velocity is dependent on the elasticity and density of the solid. Together with density testing, differences in sound velocity between precious metals, and the base metals used to counterfeit them, guarantees detection of fakes. Ultrasonic measurement calculates the distance sound waves of a known velocity travel through a gold or silver bar and echo back to the surface. This distance is compared with the measured width of the bar.
For example, a PAMP Suisse 10oz gold bar has a thickness of 8.3mm. If that bar was comprised of tungsten instead, and the instrument is pre-selected for the velocity of gold, it would return a measurement of only 5.2mm. The discrepancy represents the proportion that the sound velocity for tungsten (5170 m/s) is faster than the sound velocity for gold (3240 m/s). If the dimensions of the bullion being tested are not known then a vernier caliper or micrometer can be used to measure them. Any result that significantly varies from the known thickness warrants further investigation.