Abstract: A body (40) is provided which is inserted between an ultrasound crystal (30) and an output port (27) in a probe filled with a fluid, to centrally defocus generated ultrasound and create an output ultrasound field having limited propagation, especially for higher intensities, after passing the output port. The casing is divided into three portions (21, 22, 23), whereby a front conical portion (21) including the output port (27) and the centrally defocusing body (40) is easily replaceable. The fluid in the casing is refilled via a tube connection (12) to a fluid storage connected as a communicating vessel to the ultrasound device. Moreover, the output port of the device is provided with a thin membrane (10), preferably of replaceable latex rubber, which is attached to prevent the fluid in the casing from emerging, but at the same time enabling the effective transfer of generated ultrasound. The membrane (10) may be attached by placing an O-ring gasket in a circularly formed slot (25) on the conical portion.