A CPAP system for providing gas at positive pressure for respiratory therapy to a patient, the CPAP system comprising: an RPT device configured to supply a flow of gas at a therapeutic pressure; a patient interface forming a plenum chamber pressurizable to the therapeutic pressure, the patient interface including a seal-forming structure constructed and arranged to form a seal with a region of a patient's face surrounding an entrance to a patient's airways; an air delivery conduit configured to pass the flow of gas at the therapeutic pressure from the RPT device to the patient interface; and a vent assembly (4400, 5400) configured to provide a vent flow of gas to discharge gas exhaled by the patient from a pressurized volume to ambient, the vent assembly comprising: a base (4700, 5700) including at least one first orifice (4710, 5710) extending through the base to allow gas to be discharged from the pressurized volume to ambient along a primary vent flow path and at least one second orifice (4720, 5720) to allow gas to be discharged from the pressurized volume to ambient along a secondary vent flow path; a diffusing member (4600, 5600) provided to the base, the diffusing member configured and arranged such that the at least one first orifice is covered by the diffusing member so that vent flow passes through the diffusing member along the primary vent flow path and the at least one second orifice is not covered by the diffusing member so that vent flow bypasses the diffusing member along the secondary vent flow path; and a membrane (4800, 5800) provided to the base, wherein the membrane is structured and arranged to apportion the vent flow of gas along the primary vent flow path and the secondary vent flow path throughout respiratory therapy. an RPT device configured to supply a flow of gas at a therapeutic pressure; a patient interface forming a plenum chamber pressurizable to the therapeutic pressure, the patient interface including a seal-forming structure constructed and arranged to form a seal with a region of a patient's face surrounding an entrance to a patient's airways; an air delivery conduit configured to pass the flow of gas at the therapeutic pressure from the RPT device to the patient interface; and a vent assembly (4400, 5400) configured to provide a vent flow of gas to discharge gas exhaled by the patient from a pressurized volume to ambient, the vent assembly comprising: a base (4700, 5700) including at least one first orifice (4710, 5710) extending through the base to allow gas to be discharged from the pressurized volume to ambient along a primary vent flow path and at least one second orifice (4720, 5720) to allow gas to be discharged from the pressurized volume to ambient along a secondary vent flow path; a diffusing member (4600, 5600) provided to the base, the diffusing member configured and arranged such that the at least one first orifice is covered by the diffusing member so that vent flow passes through the diffusing member along the primary vent flow path and the at least one second orifice is not covered by the diffusing member so that vent flow bypasses the diffusing member along the secondary vent flow path; and a membrane (4800, 5800) provided to the base, wherein the membrane is structured and arranged to apportion the vent flow of gas along the primary vent flow path and the secondary vent flow path throughout respiratory therapy. a base (4700, 5700) including at least one first orifice (4710, 5710) extending through the base to allow gas to be discharged from the pressurized volume to ambient along a primary vent flow path and at least one second orifice (4720, 5720) to allow gas to be discharged from the pressurized volume to ambient along a secondary vent flow path; a diffusing member (4600, 5600) provided to the base, the diffusing member configured and arranged such that the at least one first orifice is covered by the diffusing member so that vent flow passes through the diffusing member along the primary vent flow path and the at least one second orifice is not covered by the diffusing member so that vent flow bypasses the diffusing member along the secondary vent flow path; and a membrane (4800, 5800) provided to the base, wherein the membrane is structured and arranged to apportion the vent flow of gas along the primary vent flow path and the secondary vent flow path throughout respiratory therapy.