transverse to the direction r the light source 4 is arranged in front of or behind the resonator 3 such that the resonator 3 when illuminated only with the light source 4 casts a shadow on its side facing away from the light source 4 to prevent such a shadow the ultrasound system 1 of the exemplary embodiment of the fig1 has an additional light source 8 arranged opposite the light source 4 in relation to the resonator 3 by arranging the resonator 3 between the light sources 4 8 the light sources 4 8 can also illuminate shadows produced by the respective other light source 4 8 enabling a user of the ultrasound system 1 to clearly see all sides of the resonator 3 and in particular its end 7 from all sides of the resonator 3 during the operation in particular the illuminated region extending around the resonator 3 can be referred to as a workspace a when the workspace a is illuminated in the region of the resonator 3 and in particular in the region of its end 7 an operator of the ultrasound system 1 can safely handle the ultrasound system 1 and easily visually check whether the resonator 3 is correctly placed in contact with the medium 10 fig1 shows the ultrasound system 1 with a container 9 wherein a flowable medium 10 to be treated with ultrasound is arranged in the container 9 at least the end 7 of the resonator 3 is immersed in the flowable medium 10 so as to transmit.