Figure 2. Animation demonstrating the principle by which the acoustic-resonance spectrometer operates. In the absence of a sample, excitation of the rod results in a standing wave and a consistent frequency spectrum. In the presence of a sample, a second sound path is introduced, passing through the sample and reflecting off the sample holder, and recombining with the original standing wave. This results in an entirely new wave, the interference pattern from the combination of frequencies from the standing wave and the sample wave.