Explaining what mufflers do is pretty easy, they make a vehicle run quieter. However, explaining how a muffler does this is harder than you think. Believe it or not, inside your common muffler are chambers as finely tuned as a musical instrument.
Sound is a wave formed from pulses of alternating high and low air pressure, basically air molecules in motion. In an engine, the pulses are created when exhaust valves open and high-pressure exhaust gas bursts into the exhaust system. Now comes the interesting part: It is possible to produce a sound wave that is exactly the opposite of another sound wave. If this is done right and you can get the two to hit each other, they cancel out. That’s right, the sound actually disappears.
This noise cancellation technique is not the only thing going on in your muffler. For example: the body of a muffler is generally constructed in three layers. This allows the body of the muffler to absorb some of the pressure pulses so they don’t cause the muffler body to act like a drum.
Engineers have to be careful when they design mufflers, though, because they don’t want too much “backpressure”. This subtracts a little from the power of the engine. For those that this is a big disadvantage, like high performance and sports cars, there are other types of mufflers that can be used that have minimal backpressure.
Several car manufacturers have been experimenting with high tech active noise-canceling mufflers. These systems actually incorporate microphones and speaker devices in the exhaust pipe. The speaker device is positioned in the pipe and a computer monitors the noise. By knowing some things about the length and shape of the pipes, the computer can generate a signal to cancel out exhaust noise.
Source: Bossier Chrysler Tech Department