A washout vent for a mask for use with a system for supplying breathable gas pressurized above atmospheric pressure to an airway of a mammal for treatment of sleep disordered breathing, includes a solid mask section and a vent orifice extending through a thickness of the solid mask section and adapted for gas washout. The vent orifice includes opposed side walls providing a flow passage that allows gas to flow along the opposed side walls from an interior of the mask to atmosphere in use, and the opposed side walls defining at least a portion of a conic section having a length substantially greater than its width. The vent orifice includes an orifice inlet at the interior of the mask and an orifice exit open to atmosphere, and the opposed side walls converge from the orifice inlet to the orifice exit to reduce noise of gas washout.