Explaining how a muffler does this is harder than you think. Inside your common muffler are chambers as finely tuned as a musical instrument. These chambers are designed to reflect the sound waves produced by the engine in such a way that they partially cancel themselves out. Let’s see how this works:
Sound is a wave created from pulses of alternating high and low air pressure, or air molecules in motion. In an engine, the pulses are made when exhaust valves open and high-pressure exhaust gas bursts into its exhaust system. When this happens, molecules of this gas run into the lower-pressure molecules in the pipe, resulting in them pushing on each other and moving on down the exhaust pipe. When these pressure pulses reach your ears, your eardrums vibrate back and forth to make you hear sound.
Now, it is possible to create a sound wave that is the opposite of another sound wave. If this is done correctly and you can get the two to hit each other, they’re cancelled out. That’s right, the sound disappears. Located inside the muffler is a set of tubes that will do this. According to the team at this Freeport, NY Chevrolet dealer, the result is a decrease in exhaust noise.
This noise cancellation technique isn’t the only thing happening in your muffler. There are other things that help it cut down the sound level too. For example: the body of a muffler is constructed in three layers: Two thin metal layers with a thicker, slightly insulated layer between them. This allows the muffler’s body to absorb some of the pressure pulses so they do not cause the muffler body to act like a drum. Also, the inlet and outlet pipes that go into the main chamber have small holes. These allow thousands of very small pressure pulses to bounce around in the main chamber, which cancel out each other and in addition are absorbed by the muffler’s housing.
Engineers must exercise caution when designing mufflers, though, because they don’t want too much “backpressure”. This subtracts a little from the engine’s power. This would be a huge disadvantage for some vehicles, like high performance and sports cars. However, there are other types of mufflers that can be used that have minimal backpressure. One, a “glass pack”, uses just absorption to reduce the sound. On a glass pack type muffler, the exhaust goes through a pipe that has holes. No fancy wave cancellation is involved. A layer of glass insulation surrounds this pipe, and absorbs some of the pressure pulses. Mufflers like these are used for performance applications as they’re louder than your typical muffler.
Several car makers have experimented with high tech active noise-canceling mufflers. These systems have microphones and speaker-like devices in their exhaust pipes. The speaker device is positioned in the pipe and one computer monitors both a microphone positioned before the speaker and one positioned after the speaker. By knowing some things about the shape and length of the pipes, the computer generates a complex signal to drive the speaker and cancel out exhaust noise.