A HYDROACOUSTIC TRANSDUCER IS DESCRIBED HAVING A RADIATOR IN THE FORM OF A FLEXING DISC AND AN ACOUSTIC DRIVE MEMBER FOR COUPLING A FLUID-FILLED CAVITY, THE PRESSURIZED FLUID IN WHICH IS ACOUSTICALLY MODULATED, TO THE RADIATOR. THE DRIVE MECHANISM INCLUDES A PISTON WHICH IS GUIDED IN AN OPENING IN THE HOUSING WHICH EXTENDS INTO THE CAVITY CONTAINING THE PRESSURE-MODULATED FLUID. AN ANNULAR-SEAL IS PROVIDED IN THE OPENING AND EXTENDS PARTIALLY INTO THE PISTON AND INTO THE WALL OF THE OPENING. THE WALL OF THE OPENING AND THE PISTON IS CONSTRUCTED TO PROVIDE A CLAMPING STRUCTURE SO THAT FLUID IS PREVENTED FROM LEAVING THE CAVITY WHILE THE PISTON IS PERMITTED TO VIBRATE IN RESPONSE TO THE ACOSTIC ENERGY OF THE MODULATED FLUID.