A muffler reduces the exhaust noise of a vehicle by blocking and reflecting sound waves. Inside the muffler is a series of baffled chambers and perforated barriers that break up the high-velocity sound waves as they flow through the device, reducing the overall noise level.
When an engine exhausts gas, it does so in a series of rapid, staccato bursts. These short blasts produce sound waves by compressing the air into discrete waves. When the exhaust valve is open, the gas molecules are pushed ahead and compressed into a wave, and when it closes, they become slack and sparse. As these waves travel through the chambers of the muffler, some of them strike surfaces and reflect back toward the source. When two opposite waves combine, they cancel each other out.
Mufflers are not perfect at reducing engine noise because the more exhaust gas that is blocked or redirected inside the muffler, the less power the engine has. Some of the exhaust gases must leave the tailpipe unimpeded to maintain engine power, so even a car with a muffler can be loud under the right circumstances. There are many different styles of mufflers available, each with its own level of sound suppression.