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.