A patient interface for sealed delivery of a flow of air at a continuously positive pressure with respect to ambient air pressure to an entrance to the patient's airways including at least an entrance of a patient's nares, wherein the patient interface is configured to maintain a therapy pressure in a range of about 4 cmH2O to about 30 cmH2O above ambient air pressure in use, throughout the patient's respiratory cycle, while the patient is sleeping, to ameliorate sleep disordered breathing, said patient interface comprising: a seal-forming structure to form a pneumatic seal with the entrance to the patient's airways; a positioning and stabilising structure to maintain the seal-forming structure in sealing contact with an area surrounding the entrance to the patient's airways while maintaining a therapeutic pressure at the entrance to the patient's airways; a plenum chamber pressurised at a pressure above ambient pressure in use; a connection port for the delivery of the flow of breathable gas into the patient interface; and a device positioned within a breathing chamber defined, at least in part, by the seal-forming structure and the plenum chamber when the seal-forming structure is in sealing contact with the area surrounding the entrance to the patient's airways, wherein the device divides the breathing chamber into a posterior chamber and an anterior chamber , wherein, in use, the posterior chamber is proximal to the entrance of the patient's airways relative to the anterior chamber , wherein the device comprises a plurality of apertures such that the velocity of the air in the posterior chamber is less than the velocity of the air in the anterior chamber , wherein the device is shaped and dimensioned such that a portion of an outer peripheral edge region of the device substantially conforms to an internal surface of at least one of the seal-forming structure and the plenum chamber , and wherein the patient interface comprises at least one gap between the outer peripheral edge region of the device and the internal surface of at least one of the seal-forming structure and the plenum chamber such that a portion of the flow of air can bypass the device . a seal-forming structure to form a pneumatic seal with the entrance to the patient's airways; a positioning and stabilising structure to maintain the seal-forming structure in sealing contact with an area surrounding the entrance to the patient's airways while maintaining a therapeutic pressure at the entrance to the patient's airways; a plenum chamber pressurised at a pressure above ambient pressure in use; a connection port for the delivery of the flow of breathable gas into the patient interface; and a device positioned within a breathing chamber defined, at least in part, by the seal-forming structure and the plenum chamber when the seal-forming structure is in sealing contact with the area surrounding the entrance to the patient's airways, wherein the device divides the breathing chamber into a posterior chamber and an anterior chamber , wherein, in use, the posterior chamber is proximal to the entrance of the patient's airways relative to the anterior chamber , wherein the device comprises a plurality of apertures such that the velocity of the air in the posterior chamber is less than the velocity of the air in the anterior chamber , wherein the device is shaped and dimensioned such that a portion of an outer peripheral edge region of the device substantially conforms to an internal surface of at least one of the seal-forming structure and the plenum chamber , and wherein the patient interface comprises at least one gap between the outer peripheral edge region of the device and the internal surface of at least one of the seal-forming structure and the plenum chamber such that a portion of the flow of air can bypass the device . <EOS>