Vent and vent adaptor for patient interface

A vent system for use with a patient interface during respiratory therapy of a patient with a therapy flow of gas pressurized above ambient pressure, 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 having a plurality of first orifices and a plurality of second orifices extending through the base to allow gas to be discharged to atmosphere from the pressurized volume, and the plurality of second orifices being positioned radially outward of the plurality of first orifices with respect to a central axis through the base;
• and
• a membrane positioned adjacent to the base within the pressurized volume,
• wherein the pressurized volume is in fluid communication with atmosphere through the plurality of second orifices throughout a therapeutic pressure range,
• wherein the membrane is configured such that an increase in pressure within the pressurized volume causes the membrane to restrict a first vent flow through the plurality of first orifices throughout the therapeutic pressure range, and
• wherein restriction of the first vent flow through the plurality of first orifices causes an increase in a second vent flow through the plurality of second orifices such that the vent flow through the plurality of first orifices and the plurality of second orifices is approximately constant throughout the therapeutic pressure range,
• wherein the vent housing comprises an outer wall and an inner wall, the inner wall defining an inlet for the therapy flow of gas, and
• wherein the base is positioned between the outer wall and the inner wall.