A pressure wave generator for generating a pressure wave in a medium, the pressure wave generator comprising: a movable piston having a first surface and a second surface and a longitudinal axis between the first surface and the second surface; a housing having an inner bore, a first end, and a second end, the piston being at least partially disposed within the inner bore of the housing, the piston movable within the inner bore of the housing from the first end toward the second end along a direction parallel to the longitudinal axis of the piston; a control rod extending longitudinally from a first end to a second end; a transducer slidably accommodated in the second end of the housing, the transducer configured to be coupled to the medium and adapted to convert a portion of the kinetic energy of the piston into a pressure wave in the medium upon impact of the piston with the transducer; a motive force generator (19, 20, 200, 203, 205, 210) configured to accelerate the movable piston toward the transducer, characterized by the piston comprising a guide extending along the longitudinal axis of the piston from the first surface at least partially toward the second surface, the guide having an entrance opening at the first surface, the second end of the control rod configured to be inserted in the guide and able to move within the guide without being displaced from the guide during movement of the piston within the inner bore; and a retainer having an opening in which the second end of the control rod can be inserted and secured, the retainer having a first orientation when disposed in the guide in which the opening faces a wall of the guide so that a first cross-sectional area of the retainer is smaller than a cross-sectional area of the entrance opening, thereby permitting the retainer to be inserted into the guide, the retainer movable to a second orientation when disposed in the guide in which the opening faces the entrance opening so that a second cross-sectional area of the retainer is larger than the cross-sectional area of the entrance opening, thereby inhibiting the retainer from being displaced from the guide, wherein on impact of the piston and the transducer, the second end of the control rod moves within the guide toward the second surface of the piston. a movable piston having a first surface and a second surface and a longitudinal axis between the first surface and the second surface; a housing having an inner bore, a first end, and a second end, the piston being at least partially disposed within the inner bore of the housing, the piston movable within the inner bore of the housing from the first end toward the second end along a direction parallel to the longitudinal axis of the piston; a control rod extending longitudinally from a first end to a second end; a transducer slidably accommodated in the second end of the housing, the transducer configured to be coupled to the medium and adapted to convert a portion of the kinetic energy of the piston into a pressure wave in the medium upon impact of the piston with the transducer; a motive force generator (19, 20, 200, 203, 205, 210) configured to accelerate the movable piston toward the transducer, characterized by the piston comprising a guide extending along the longitudinal axis of the piston from the first surface at least partially toward the second surface, the guide having an entrance opening at the first surface, the second end of the control rod configured to be inserted in the guide and able to move within the guide without being displaced from the guide during movement of the piston within the inner bore; and a retainer having an opening in which the second end of the control rod can be inserted and secured, the retainer having a first orientation when disposed in the guide in which the opening faces a wall of the guide so that a first cross-sectional area of the retainer is smaller than a cross-sectional area of the entrance opening, thereby permitting the retainer to be inserted into the guide, the retainer movable to a second orientation when disposed in the guide in which the opening faces the entrance opening so that a second cross-sectional area of the retainer is larger than the cross-sectional area of the entrance opening, thereby inhibiting the retainer from being displaced from the guide, wherein on impact of the piston and the transducer, the second end of the control rod moves within the guide toward the second surface of the piston.