A gas diffusion device used in conjunction with a flow of gas from a conduit utilizing a sheet having a peripheral edge defining a surface inwardly from the edge. The surface possesses a first zone, and a second zone between the first zone and the peripheral edge. A first plurality of apertures through the sheet lie in the first zone. A second plurality of apertures through the sheet lie in the second zone. The percentage of open area of the first plurality of apertures relative to the area of the first zone is less than the percentage of open area due to the second plurality of apertures through the second zone.