Aircraft, environmental control systems, and air diffusers that may be used to create a quiet, comfortable environment within an aircraft cabin are disclosed. The air diffusers include a duct, a grille, and a porous flow conditioner positioned at least partially within an outlet plenum of the duct. The porous flow conditioner is configured to create backpressure and to reduce a velocity of air flowing out of the porous flow conditioner, relative to air flow unaffected by the porous flow conditioner. The porous flow conditioner may be configured to reduce noise generated by air flow through the air diffuser assembly, relative to air flow unaffected by the porous flow conditioner. The porous flow conditioner may be configured to reduce noise generated by air flow downstream of the porous flow conditioner.