Silencers are typically made from aluminum-coated steel plate or stainless steel plate. They usually consist of a resonance chamber, an expansion chamber, and a set of porous pipes. Some silencers also contain heat-resistant sound-absorbing material inside, which is often glass fiber or asbestos. Exhaust gases flow through the porous pipes into the expansion and resonance chambers, where they continuously change direction, gradually reducing and attenuating their pressure and pressure fluctuations, dissipating their energy, thereby reducing the exhaust noise.