The design of the straight through silencer allows the gas to pass generally straight through a perforated steel tube. This tube is housed in an outer casing with sound absorbent material between the two sides of the outer casing. The vibrating exhaust gas passes through the holes in the tube and penetrates in to the absorbent material, usually of wire or glass wool, where the direction of gas flow is changed. This process lowers the frequency of gas vibrations, which lowers the pitch of the sound. The absorption silencer sounds different though it is not necessarily noisier than the baffle type. There is little backpressure with this design of silencer and it is used on some heavy vehicles.