Muffler

[4] The noise of the hot exhaust gas exiting the engine can be abated by a series of passages and chambers lined with roving fiberglass insulation and/or resonating chambers harmonically tuned to cause destructive interference, wherein opposite sound waves cancel each other out.

[6] The exhaust system needs negative pressure waves so they help empty the cylinder of gases.

[citation needed] A side effect of noise reduction is the restriction of the exhaust gas flow, which creates back pressure, which can decrease engine efficiency.

[9] On May 18, 1905, the state of Oregon passed a law that required vehicles to have "a light, a muffler, and efficient brakes".

[10] The legality of altering a motor vehicle's original equipment exhaust system varies by jurisdiction; in many developed countries such as the United States,[11] Canada,[12] and Australia,[13] such modifications are highly regulated or strictly prohibited.