than 1 0 is required above zone c by means of creating velocity profiles the situation is generated where the various materials will tend to concentrate at different levels above the perforated deck according to their aerodynamic characteristics basically into two levels undesired heavies at the lower level and desired lights at the higher level the vibratory action of the entire unit will also encourage heavies to sink down through the floating mass of lights the input material mixture will contain an expected range in the proportion of undesired heavies this range of expected proportion is taken into account when designing the deck profile and the extent of each perforated zone these proportions are normally expressed in mass terms and 2 and 5 objectionable heavies by mass would not be untypical in a tobacco cut rolled stem process however for this application the proportion range may be expressed in terms of volumetric proportions when in a vibrated situation this is because the desired and undesired materials have different specific volumes and it is these volumes which are important if for example it is found that the proportion of undesired heavies on a volumetric transport basis is 4 to 10 then the situation in fig1 may be used where the cross sectional area above all zone a regions corresponds approximately to the minimum expected objectionable product volume while that above a and b corresponds approximately.