All resonant frequencies above the fundamental resonance. Because of the varying cross-sectional dimensions of ultrasonic resonators, the overtones are not normally integer multiples (2, 3, 4, ... -- i.e., harmonics) of the frequency of the fundamental resonance. For example, if the fundamental resonance has a frequency of 1000 Hz, then the overtones might have frequencies of 1720 Hz, 2364 Hz, 3379 Hz, etc. This term is often applied to a particular mode -- e.g., the overtones of the axial mode, the overtones of the flexural mode, the overtones of the radial mode, etc.

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