A modulator is disclosed for creating a pressure pulse in a fluid-filled well. The modulator comprises a tool body on which a plurality of extendable arms are mounted. The arms may be retracted into a stowed position substantially adjacent the tool body, or may be extended to meet the wall of the borehole. The arms are preferably resilient bowsprings that may be flexed outwards from the tool by means of an actuator pushing on at least one end of the springs. A flexible valve sleeve or bag is suspended between the arms and cooperates with a valve mounted adjacent the sleeve on the tool body. The valve sleeve creates a fluid-flow path through the valve, and in operation, the valve closes one end of the valve sleeve to create the pressure pulse. Sealing of the valve sleeve against the wall of the well is a result of the fluid pressure inflating the sleeve against the well wall. As a result, sealing takes place over an extended area of the bag and is dynamically responsive to changes in fluid flow or pressure.