A patient interface for providing respiratory therapy to a patient with a therapy flow of gas pressurized at a therapeutic pressure above ambient pressure, the patient interface comprising: a seal-forming structure configured to contact and seal against the patient's face during use; a plenum chamber joined to the seal-forming structure and configured to be pressurized to the therapeutic pressure by the therapy flow of gas during use; a positioning and stabilising structure including at least one tie to secure the patient interface on the patient's head in use; and a vent system , the vent system providing 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 (13403; 13406) having a plurality of first orifices extending through the base (13403; 13406) to allow gas to be discharged to atmosphere from the pressurized volume; 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 (13403; 13406), wherein the pressurized volume is in fluid communication with atmosphere through the plurality of first orifices and the plurality of second orifices throughout a therapeutic pressure range, wherein the membrane is elastically deformable due to pressure within the pressurized volume to apportion the vent flow between the plurality of first orifices and the plurality of second orifices throughout the therapeutic pressure range, and wherein the membrane is not attached to the vent housing such that the membrane is freely movable towards and away from the base (13403; 13406). a seal-forming structure configured to contact and seal against the patient's face during use; a plenum chamber joined to the seal-forming structure and configured to be pressurized to the therapeutic pressure by the therapy flow of gas during use; a positioning and stabilising structure including at least one tie to secure the patient interface on the patient's head in use; and a vent system , the vent system providing 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 (13403; 13406) having a plurality of first orifices extending through the base (13403; 13406) to allow gas to be discharged to atmosphere from the pressurized volume; 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 (13403; 13406), a vent housing comprising a base (13403; 13406) having a plurality of first orifices extending through the base (13403; 13406) to allow gas to be discharged to atmosphere from the pressurized volume; 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 (13403; 13406), wherein the pressurized volume is in fluid communication with atmosphere through the plurality of first orifices and the plurality of second orifices throughout a therapeutic pressure range, wherein the membrane is elastically deformable due to pressure within the pressurized volume to apportion the vent flow between the plurality of first orifices and the plurality of second orifices throughout the therapeutic pressure range, and wherein the membrane is not attached to the vent housing such that the membrane is freely movable towards and away from the base (13403; 13406). <EOS>