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How Mufflers Work

Mufflers cancel out most of an engine's noise.
Mufflers cancel out most of an engine's noise.




But first, we need to know a little about sound. 

Where Does the Sound Come From?





It turns out that it is possible to add two or more sound waves together and get less sound. Let's see how.

How Can You Cancel Out Sound?



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How sound waves add and subtract


Inside a Muffler



A chamber called a resonator is connected to the first chamber by a hole. The resonator contains a specific volume of air and has a specific length that is calculated to produce a wave that cancels out a certain frequency of sound. How does this happen? Let's take a closer look ...

The Resonator

When a wave hits the hole, part of it continues into the chamber and part of it is reflected. The wave travels through the chamber, hits the back wall of the muffler and bounces back out of the hole. The length of this chamber is calculated so that this wave leaves the resonator chamber just after the next wave reflects off the outside of the chamber. Ideally, the high-pressure part of the wave that came from the chamber will line up with the low-pressure part of the wave that was reflected off the outside of the chamber wall, and the two waves will cancel each other out.

The animation below shows how the resonator works in a simplified muffler.

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Waves canceling inside a simplified muffler


Some cars, especially luxury cars where quiet operation is a key feature, have another component in the exhaust that looks like a muffler, but is called a resonator. This device works just like the resonator chamber in the muffler -- the dimensions are calculated so that the waves reflected by the resonator help cancel out certain frequencies of sound in the exhaust.

There are other features inside this muffler that help it reduce the sound level in different ways. The body of the muffler is constructed in three layers: Two thin layers of metal with a thicker, slightly insulated layer between them. This allows the body of the muffler to absorb some of the pressure pulses. Also, the inlet and outlet pipes going into the main chamber are perforated with holes. This allows thousands of tiny pressure pulses to bounce around in the main chamber, canceling each other out to some extent in addition to being absorbed by the muffler's housing.

Backpressure and Other Types of Mufflers

The exhaust from a NASCAR race car: There are no mufflers here, because reducing backpressure is the name of the game.

One important characteristic of mufflers is how much backpressure they produce. Because of all of the turns and holes the exhaust has to go through, mufflers like those in the previous section produce a fairly high backpressure. This subtracts a little from the power of the engine.

There are other types of mufflers that can reduce backpressure. One type, sometimes called a glass pack or a cherry bomb, uses only absorption to reduce the sound. On a muffler like this, the exhaust goes straight through a pipe that is perforated with holes. Surrounding this pipe is a layer of glass insulation that absorbs some of the pressure pulses. A steel housing surrounds the insulation.

Diagram of glass pack muffler
Diagram of glass pack muffler


Active Noise-Canceling Mufflers

There have been a few experiments with active noise-canceling mufflers, especially on industrial generators. These systems incorporate a set of microphones and a speaker.

The speaker is positioned in a pipe, which wraps around the exhaust pipe so that the sound from the exhaust comes out in the same direction as the sound from the speaker. A computer monitors a microphone positioned before the speaker and one positioned after the speaker. By knowing some things about the length and shape of the pipes, the computer can generate a signal to drive the speaker. This can cancel out much of the sound coming from the generator. The downstream microphone lets the computer know how well it is doing so it can make adjustments if needed.

For more information on mufflers, sound and related topics, check out the links on the next page.