Abstract: In a rotating disc wood chip refiner having a number of refiner plates with a grid of radial bars and slots with a multiplicity of dams in the slots, the dams are spaced in outwards decreasing increments forming a continuous series of resonant cavities, and the radial bars are radially and uniformly skewed backwards at a radial angle of about 1.5 degrees to disc rotation. These improvements provide a reduced energy input by means of a resonating cavity flow regime, together with a low fluid-dynamic drag radial bar profile and a pressure recovery radial slot profile.