The rate at which an event recurs. When associated with vibration, the assumption is made that the event occurs at regular intervals (i.e., it is periodic). In the low frequency ultrasonic range, frequency is usually expressed in Hertz (Hz) or kiloHertz (kHz).
When applied to ultrasonic resonators, "frequency" generally refers to the resonant frequency of a particular vibration mode. If not otherwise specified, "frequency" refers to the frequency of the primary mode of the resonator. For example, if the primary mode of a horn is the axial mode, then the statement "The horn frequency is 19852 Hz." indicates that the frequency of the axial (primary) mode is 19852 Hz.