All else equal, two cash made from two extraordinary metals will make two distinctive sounds. When a coin or bar is held effectively and tapped, the whole object receives excited and vibrates in more than one distinct patterns regarded as vibration modes, every of these vibration modes creates a special height frequency and decay rate.
The StackerTools App listens for these top frequencies (and standard decay) and notifies the User if the examined object is a pass, fail, or other.