MUFFLER

also called Silencer, device through which the exhaust gases from an internal-combustion engine are passed to suppress the airborne noise of the engine. To be efficient as a sound silencer, a muffler must decrease the velocity of the exhaust gases and either absorb the sound waves or cancel them by interference with reflected waves coming from the same source. A typical sound-absorbing material used in a muffler is a thick layer of fine fibres; the fibres are caused to vibrate by the sound waves, thus converting the sound energy to heat. Muffler configurations that cancel sound waves by interference generally separate the waves into two components that follow different paths and then come together again out of phase (out of step). Noise flow through a typical muffler In the typical muffler shown schematically in the illustration, the heavy arrows indicate the gas flow, and the light arrows the acoustic flow. The chambers B and C are known as resonators and are of such dimensions that they cancel sound waves of specified frequencies. The discharge tube D is surrounded by small annular (ringlike) chambers; sound enters these chambers through small holes in the discharge tube and is absorbed. Mufflers of the straight-through type have a single tube with small holes connecting with annular chambers that are frequently stuffed with a sound-absorbing material.