The invention concerns a device for treating body tissue and/or crushing calculi by means of acoustic energy, such as shock, pressure or ultrasonic waves. The device comprises: a sound-generating unit which produces acoustic waves; optionally a downstream focusing unit for the acoustic waves; and a transmission medium for guiding the acoustic waves generated by the sound-generating unit to the body. The device according to the invention is characterized in that the shape of the interface and/or the conditions at the interface between the transmission medium and the body are such that energy is transmitted by the sound-generating unit into the body without substantial deviations from the desired extent of diffusion.