This invention relates to .beta.-SiC foam parts with a specific surface area preferably equal to at least 5 m.sup.2/g and with at least two zones A and B with a different cellular porosity distribution, wherein the parts were made by chemical transformation of a porous precursor medium comprising at least two blocks A' and B', each having a different cellular porosity distribution, and in that the at least two zones A and B are derived from the chemical transformation of the two blocks A' and B'. This foam, optionally after deposition of an active layer, may be used as a filter medium in cartridges designed for the purification of exhaust gases. The invention also relates to manufacturing processes for preparing such a filter medium.