A patient interface configured to provide respiratory therapy to a patient with a therapy flow of gas pressurized above ambient pressure throughout the patient's respiratory cycle, the patient interface comprising: a seal-forming structure configured to seal against the patient's face in use to provide the therapy flow of gas to the patient's airways; a plenum chamber joined to the seal-forming structure; a positioning and stabilising structure configured to secure the seal-forming structure on the patient in use; and a vent system configured to provide a vent flow of gas to discharge gas exhaled by the patient from a pressurized volume, the vent flow being continuous during the respiratory therapy, the vent system comprising: a vent housing comprising a base having at least one first orifice extending through the base to allow gas to be discharged to atmosphere from the pressurized volume, wherein the base comprises an inner base and an outer base; a plurality of second orifices to allow gas to be discharged to atmosphere from the pressurized volume; and a membrane positioned adjacent to the base, wherein a plurality of lateral membrane supports on the outer base are configured to prevent the membrane from covering the plurality of second orifices; wherein the pressurized volume is in fluid communication with atmosphere through the at least one first orifice and the at least one second orifice throughout a therapeutic pressure range, wherein the membrane is constructed from an elastically deformable material and is configured to elastically deform due to pressure within the pressurized volume throughout the therapeutic pressure range such that as pressure within the pressurized volume increases above a threshold the membrane is deformed towards the base to reduce a proportion of the vent flow passing through the at least one first orifice and to increase a proportion of the vent flow passing through the at least one second orifice, and wherein the membrane is not attached to the vent housing such that the membrane is freely movable towards and away from the base. a seal-forming structure configured to seal against the patient's face in use to provide the therapy flow of gas to the patient's airways; a plenum chamber joined to the seal-forming structure; a positioning and stabilising structure configured to secure the seal-forming structure on the patient in use; and a vent system configured to provide a vent flow of gas to discharge gas exhaled by the patient from a pressurized volume, the vent flow being continuous during the respiratory therapy, the vent system comprising: a vent housing comprising a base having at least one first orifice extending through the base to allow gas to be discharged to atmosphere from the pressurized volume, wherein the base comprises an inner base and an outer base; a plurality of second orifices to allow gas to be discharged to atmosphere from the pressurized volume; and a membrane positioned adjacent to the base, wherein a plurality of lateral membrane supports on the outer base are configured to prevent the membrane from covering the plurality of second orifices; wherein the pressurized volume is in fluid communication with atmosphere through the at least one first orifice and the at least one second orifice throughout a therapeutic pressure range, wherein the membrane is constructed from an elastically deformable material and is configured to elastically deform due to pressure within the pressurized volume throughout the therapeutic pressure range such that as pressure within the pressurized volume increases above a threshold the membrane is deformed towards the base to reduce a proportion of the vent flow passing through the at least one first orifice and to increase a proportion of the vent flow passing through the at least one second orifice, and wherein the membrane is not attached to the vent housing such that the membrane is freely movable towards and away from the base. a vent housing comprising a base having at least one first orifice extending through the base to allow gas to be discharged to atmosphere from the pressurized volume, wherein the base comprises an inner base and an outer base; a plurality of second orifices to allow gas to be discharged to atmosphere from the pressurized volume; and a membrane positioned adjacent to the base, wherein a plurality of lateral membrane supports on the outer base are configured to prevent the membrane from covering the plurality of second orifices; wherein the pressurized volume is in fluid communication with atmosphere through the at least one first orifice and the at least one second orifice throughout a therapeutic pressure range, wherein the membrane is constructed from an elastically deformable material and is configured to elastically deform due to pressure within the pressurized volume throughout the therapeutic pressure range such that as pressure within the pressurized volume increases above a threshold the membrane is deformed towards the base to reduce a proportion of the vent flow passing through the at least one first orifice and to increase a proportion of the vent flow passing through the at least one second orifice, and wherein the membrane is not attached to the vent housing such that the membrane is freely movable towards and away from the base.