Abstract: The diffuser is constructed primarily of lightweight, roll formed, channeled sheet metal bent into a square by cutting the top and bottom margins at the bends. Each of the corners is then riveted. The top wall is molded fiberglass which can be easily cut to form a central opening of different diameters. The figerglass is held between the bent margins of the sidewall and a bracket riveted to the sidewall. The bottom wall is perforated and riveted to the sidewall. A radial deflector having a central portion and arms extending outward is riveted through one of the perforations in the bottom wall. Each arm has a flat portion and bent up portion. The flat portion is spaced from the bottom wall by means of small resilient tabs.