then pass through a flow path in medium 14 between the end faces with medium 14 trapping dpm in the process after having passed through the medium the treated exhaust exits the casing through an outlet port receiver 12 comprises a conical shaped inner wall 24 one end of medium 14 is inserted upright into receiver 12 through the wider open end 26 of wall 24 to place the perimeter of the medium at end 22 in abutment with wall 24 the narrower open end 28 of wall 24 opposite end 26 is communicated to an entrance 30 of collector 16 through a passageway 32 air pulse delivery system 18 comprises a device for creating percussion air pulses that are capable of dislodging trapped particulate matter from medium 14 fig1 shows only the portion of system 18 that directly associates with medium 14 that portion comprises a movable conduit 34 that includes a conical nozzle 36 at one end the opposite end that is not shown connects to the device that creates the percussion pulses for cleaning medium 14 nozzle 36 is placed against end face 20 and moved manually across the end face while percussion pulses are delivered through conduit 34 and nozzle 36 to prevent particulates from being blown into the air from end face 20 an encasement bag 38 is associated with conduit 34 and medium 14 bag 38 comprises a flexible imperforate sleeve of suitable material one end of the sleeve is sealed around the outside of conduit 34 and the opposite end around the outside of medium.