The present invention relates to an elbow assembly for use with a patient interface during respiratory therapy of a patient with a therapy flow of gas pressurized above ambient pressure. The elbow assembly provides a vent flow of gas to discharge gas exhaled by the patient from a pressurized volume within the elbow assembly. The vent flow is continuous during the respiratory therapy. The elbow assembly comprises an elbow having a first end configured to be connected to an air delivery tube and a second end opposite the first end; and a vent housing connected to the second end and comprising a base having a plurality of inner orifices extending through the base and configured to allow gas to be discharged to atmosphere from the pressurized volume and a plurality of outer orifices positioned radially outward of the plurality of inner orifices, extending through the base, and configured to allow gas to be discharged to atmosphere from the pressurized volume. The elbow assembly further comprises a membrane constructed from an elastically deformable material, the membrane being positioned parallel and adjacent to the base, in an undeformed state, to allow a first portion of the vent flow to discharge to atmosphere through the plurality of inner orifices and a second portion of the vent flow to discharge to atmosphere through the plurality of outer orifices throughout a therapeutic pressure range. The membrane is elastically deformable due to pressure within the pressurized volume to apportion the first portion of the vent flow and the second portion of the vent flow between the plurality of inner orifices and the plurality of outer orifices, respectively, depending on the pressure within the pressurized volume throughout the therapeutic pressure range.