In a control system and method for operating an ultrasonic liquid delivery device, an ultrasonic waveguide, separate from the housing, is disposed at least in part within an internal chamber of the housing to ultrasonically energize liquid prior to the liquid being exhausted from the housing through an exhaust port. An excitation device is operable to ultrasonically excite the waveguide and a control system controls operation of the liquid delivery device between an excitation mode in which the excitation device is operated at an excitation frequency to excite the ultrasonic waveguide and a ring down mode in which the excitation device is inoperable to excite the waveguide such that the waveguide rings down. The control system monitors the ring down and is responsive to the ring down to adjust the excitation frequency of the excitation device in the excitation mode thereof.