Abstract: A displacement compressor has a housing defining a chamber and having an inlet and an outlet for the chamber. A pair of meshing helical rotors are rotatably supported about parallel axes in the chamber, so that a compressible fluid may be pumped from the inlet to the outlet. A recess in the wall of the housing is positioned in the chamber and around the outlet and has a substantially trapezoidal shape, in plan. The non-parallel edges of the substantially trapezoidal shape are angled with respect to the axial direction of the rotors, preferably by an angle which is substantially equal to the twist angle of the rotors. Noise producing pressure pulsations at the outlet are thereby reduced.