NOVELTY - The system (1) has a water duct (14) extending from a sea side (10) to a land side (12), such that the duct is filled with liquid during use and the liquid in a free surface chamber (16) and the duct forms a mass-spring system. The chamber is delimited by front and back walls (28, 30). An energy conversion device (18) i.e. linear generator (20), converts fluid-dynamic energy of the liquid or gas into a different form of energy. A displacement unit (32) displaces the back wall, such that size of a free surface area is adjustable for adjustment of a natural period of the mass-spring system.    USE - Wave energy conversion system.    ADVANTAGE - The free surface chamber can be easily designed by positioning the displaceable wall opposite an end of the water duct, such that the losses due to turbulence near an opening of the water duct into the free surface chamber are minimized for all positions of the displaceable wall. The moveable back wall is not adjusted in view of every individual wave arriving at the sea side of the conversion system, so as to avoid energy losses caused by displacing the displaceable back wall. The displacement unit displaces the back wall, such that size of the free surface area is adjustable for adjustment of the natural period of the mass-spring system, thus achieving a wave energy conversion system with higher efficiency. The system can absorb one whole wave under maximum design conditions.    DETAILED DESCRIPTION - The liquid is sea water, and the back wall is moveable. An INDEPENDENT CLAIM is also included for a method for converting wave energy.    DESCRIPTION OF DRAWING(S) - The drawing shows a schematic view of a wave energy conversion system with a hinged parallelogram and a linear generator.    Wave energy conversion system (1)    Sea side (10)    Land side (12)    Water duct (14)    Free surface chamber (16)    Energy conversion device (18)    Linear generator (20)    Front wall (28)    Back wall (30)    Displacement unit (32)