A patient interface for sealed delivery of a flow of air to ameliorate sleep disordered breathing may include: 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; 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, wherein the device divides the breathing chamber into a posterior chamber and an anterior chamber, and wherein the device comprises a plurality of apertures such that turbulence of the air in the posterior chamber is less than turbulence in the air in the anterior chamber.