A muffler (or silencer in British English) is a device for reducing the amount of noise emitted by a machine. On internal combustion engines, the engine exhaust blows out through the muffler. The internal combustion engine muffler or silencer was originally invented by Milton O. Reeves.

Dual exhaust pipes attached to a car's muffler

Mufflers are typically installed along the exhaust pipe as part of the exhaust system of an internal combustion engine (of a vehicle, or stationary) to reduce its exhaust noise. The muffler accomplishes this with a resonating chamber, which is specifically tuned to cause destructive interference, where opposite sound waves cancel each other out. Catalytic converters also often have a muffling effect. The effect is mainly generated largely by restriction, rather than by cancellation.

Advantages

Mufflers that reduced backpressure relative to earlier models became increasingly available in the late 20th century, and resulted in increased engine efficiency, performance, power output, and simultaneously decreased overall wear and tear on the engines' components, as well as sound to levels in compliance with the law.

Mini-mufflers

A mini-muffler (also known as a "hotdog" in the auto industry) can be put in place of a normal muffler in a car exhaust system to "enhance" the sound of the exhaust for a car enthusiast. These are smaller, cost usually about half the price of a normal muffler and work in a similar way, but allowing more sound through. Changing the muffler / mini-muffler / catalytic converter combination can change the sound of a car's exhaust system considerably. Deliberately removing a muffler from a "factory" setup or using a muffler that is defective due to rusting risks the resulting noise being over government approved noise levels, and possible action by law enforcers. Nevertheless some motorcycle owners remove their mufflers just to make them louder.