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. The washout vent includes a vent orifice adapted for gas washout. The orifice includes one or more features that contribute to noise reduction, such as a channel with converging walls towards atmosphere, and/or a channel having one or more walls provided with surface treatment and/or contouring, such as roughening and/or scalloped portions.
