chamber does not result in too large a reduction of pressure in said energy storage when the shock wave has been generated the connections between working chamber 42 and energy storage are closed and the pressure in the working chamber is lowered by opening a connection 46 between working chamber 42 and a pressure reservoir 48 the pressure reservoir is substantially pressure relieved thereafter the connection between working chamber 42 and reservoir 48 is closed and a new stroke may by performed the working chamber is thus pressure relieved or substantially pressure relieved at the beginning of the next pulse generation when a connection between energy storage 43 and working chamber 42 is opened a part of this wave will when the pressure medium wave reaches the working chamber 42 reflect back to the energy storage 43 as a negative pressure wave which in the energy storage is re reflected whereat a new positive wave directed towards the working chamber arises this process will continue until the difference in pressure between energy storage and working chamber has leveled out by varying the distances i e the length of the channels 44 46 and the time difference between the opening of the various channels 44 46 these pressure waves and pressure reflections may be utilized to form the pressure build up and thereby the form of the shock wave according to the present invention the regulation is as before performed by a regulation device 49 but instead of regulating the reduction of pressure in a counter pressure chamber the way in which the respective channels 44 46 are opened is now regulated i e which channel is opened first and.