A device for processing workpieces uses ultrasound, with an resonant system comprising an ultrasound generator, an ultrasound sonotrode, and an anvil, wherein a workpiece is processed between the anvil and the ultrasound sonotrode. The ultrasound generator comprises a regulation means which has a regulation member connected upstream of the ultrasound generator to receive a feedback signal from the resonant system and to generate a regulation variable which is supplied to the ultrasound generator. A connecting point is provided between the regulation member and the ultrasound generator, at which the regulation variable of the regulation member is linked to a process variable from the processing procedure.