Apparatus for generating an acoustic pulse in a medium including an upstanding cylindrical housing within which an impact piston retained in the upper end of housing by an electrically controlled latch is driven downwardly by compressed gas to strike a target plate positioned adjacent the bottom of the housing. The compressed gas is provided by high volume accumulators interconnected with large area ports penetrating the sidewall of the housing near its top. The tapered upper portion of the piston presses against static seals mounted on the correspondingly tapered housing bore above and below these ports. This taper enables the compressed air to exert a positive break away force on the piston. Quick release of the latch triggers the downward thrust of the piston to deliver a blow against the target plate.