A CPAP system for respiratory therapy includes an RPT device configured to supply a flow of gas at a therapeutic pressure, a patient interface, an air delivery conduit configured to pass the flow of gas from the RPT device to the patient interface, and a vent assembly configured to provide a vent flow of gas to discharge gas exhaled by the patient from a pressurized volume to ambient. The vent assembly includes a base including at least one first orifice to allow gas to be discharged along a primary vent flow path and at least one second orifice to allow gas to be discharged along a secondary vent flow path, a diffusing member provided to the base, and a membrane provided to the base. The membrane is configured to apportion the vent flow of gas along the primary vent flow path and the secondary vent flow path throughout respiratory therapy.