Attenuation of exhaust systems refers to the reduction in sound level, measured in decibels (dBAs), between the same engine with and without a muffler. Beyond decibels, the quality of sound, as perceived by people in close proximity, is also important, so mufflers can be designed to provide different sound quality characteristics.
Traditional mufflers use tubes, baffles, expansion chambers and insulation to help control noise. Most current applications include emissions reduction devices that contribute to sound attenuation; however, in certain applications, additional sound attenuation beyond emissions systems is needed.
Engine manufacturers generally expect mufflers to provide a reduction in engine exhaust sound levels of between 15 to 25 dBA. The overall noise level of a vehicle or piece of stationary equipment is the actual regulated value and is typically measured while driving past a microphone at a specific speed and distance. Maximum noise levels are generally regulated by national governmental authorities, although some local limits may also apply.