The invention relates to a method for controlling an ultrasonic unit f an ultrasonic cleaning system with at least one control unit, one generator unit and one hand-held unit containing the ultrasonic unit and one switching module provided for starting the ultrasonic unit, wherein mechanical oscillations can be generated via the ultrasonic unit and transmitted to an instrument that is mechanically connected to the ultrasonic unit. In a preferred embodiment, the resonant frequency required for operation of the ultrasonic unit in resonance is determined by a control routine executed in the control unit, and a control signal (ss) based on the determined resonant frequency is generated in the generator unit and is sent to the ultrasonic unit for operation of the ultrasonic unit in resonance.