A marine vibratory sound source for producing swept or pulse coded signals in a body of water and within geological structures beneath a body of water, primarily for seismic exploration of the strata beneath the bottom of the water. More particularly, a high power, servo controlled hydraulic, sound source that delivers fluid alternatively to first and second actuator pistons to move the actuator pistons to a forward and an aft position thereby axially vibrating a piston shaft having cone-shaped pistons for vibrationally moving water out and in through a plurality of ports for propagating vibratory sound. The sound source having a position sensor to determine a center point of travel and by tracking the position sensor air pressure in hollow stationary cone-shaped bulkheads is modulated to maintain the travel of the cone-shaped pistons at an equal distance in the fore and aft directions from the determined center point.