Fundamental resonance
The lowest resonance of a vibrating system. This resonance is not necessarily the primary resonance. For an ultrasonic stack, this resonance is most often a flexural resonance.
This term is often applied to a particular mode -- e.g., the fundamental resonance of the axial mode, the fundamental resonance of the flexural mode, the fundamental resonance of the radial mode, etc. See Kinsler, pp. 46, 53 - 54