the airflow 40 near the open top end 48 the airflow 40 hits the plate 36 directly however the air reaching a bottom region of the plate 36 has to travel a considerable distance and be progressively turned by the plenum wall 46 as a consequence the airflow 40 reaching a bottom part of the perforated plate 36 is substantially attenuated also the angle of the airflow 40 reaching the plate 36 is influenced by the shape of a free space between the plenum 32 and the plate 36 generally the airflow 40 becomes more inclined with respect to the plate 36 toward a bottom region of the plate 36 the airflow through a hole 38 located at a specific point of the plate 36 can be evaluated by considering the portion of the airflow at the hole that is normal to the plate 36 at that point the airflow through a specific hole 38 therefore depends on the magnitude and angle of the airflow reaching that hole or in other words on the location of that hole in the plate 36 thus with equally distributed holes 38 all having the same surface area the airflow reaching the inlet 34 would have a substantially asymmetrical distribution with the airflow becoming generally progressively weaker toward a bottom end of the inlet 34 in order to correct the airflow distribution an effective area of the holes 38 is varied around the plate 36 so that the portion of the airflow 40 that blocked by the plate 36 is greater.