The invention relates to a method and a device for introducing ultrasound into a flowable medium using a sonotrode, wherein the flowable medium is not in direct contact with the sonotrode. Disclosed is a method comprising the following steps: placing a film (8) on the sonotrode (4) in such a way that the contact force by means of which the film (8) is pressed on the sonotrode (4) is always so great that the film (8) follows the lifting motions of the sonotrode (4) in the corresponding frequency and amplitude; applying ultrasound power through the film (8) into the medium (2) and transmitting the wear phenomena onto the film (8).