A method and devices comprise a low frequency high energy ultrasound system having at least one sonotrode projecting into a reactor vessel through which the liquid passes via at least one inlet orifice and at least one outlet orifice. To avoid cavitation at the sonotrode, in a close region of the oscillation-transducing sonotrode surface a pressure/amplitude combination close to or above the pressure-amplitude characteristic line is generated at which considerably reduced or no cavitation occurs and in the adjacent region in the vessel at least in a region and at least at times a pressure/amplitude combination is maintained below the pressure-amplitude characteristic line at which cavitation is generated. A device has an inlet orifice arranged such that the liquid impacts directly onto the oscillation-transducing sonotrode surface, and is shaped that in the close region of the oscillation-transducing sonotrode surface a pressure close to or above the pressure-amplitude characteristic line prevails.