Car mufflers…more complicated than you think
Explaining what mufflers do is easy -they make an engine run quieter. Explaining how a muffler does this is a little harder.
Believe it or not, inside your common muffler are chambers as finely tuned as musical instruments. These chambers are specifically designed to reflect the sound waves produced by the engine in such a way that they partially cancel themselves out.
This makes “quiet”. Let’s take a look at how this works:
First, what is “sound” exactly?
Sound is a pressure wave formed from pulses of alternating high and low air pressure. In an engine, pulses are created when exhaust valves open and bursts of high-pressure gas blast into the exhaust system. The molecules in this gas collide with the lower-pressure molecules in the pipe, causing them to push on each other. They in turn stack up on the molecules a little further down the pipe, leaving an area of low pressure behind. In this way, the sound wave makes its way down the pipe. When these pressure pulses eventually reach your ears, your eardrums vibrate back and forth and your brain interprets this motion as sound.
You can cancel out sound waves
Now comes the cool part: It is possible to produce a sound wave that is exactly the opposite of another sound wave, and if the two hit each other, they cancel out. Located inside the muffler is a set of tubes that are designed exactly to do this. The result is a dramatic decrease in sound.
Plus, there is “muffling”
There are other features inside a muffler that help it reduce the sound level. The body of a muffler is generally 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 in addition to being absorbed by the muffler’s housing.
But then there’s backpressure
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 just discussed produce a fairly high backpressure. This subtracts a little from the power of the engine and thus makes it a little bit less efficient.
There are other types of mufflers that are designed to reduce backpressure. One type, sometimes called a glass pack, uses only absorption to reduce the sound. On a muffler like this, the exhaust goes straight through a pipe that is perforated with small holes. No wave cancellation is involved. Surrounding this pipe is a layer of glass insulation that absorbs some of the pressure pulses. Glass pack mufflers are generally used for performance applications as they are louder than standard mufflers.
High Tech Mufflers
Several car manufacturers are experimenting with active noise-canceling mufflers. These systems incorporate microphones and a speaker-like devices. The speaker is positioned in the exhaust pipe and a computer monitors both the microphone positioned before the speaker and one positioned after the speaker. By knowing some things about the length and shape of the pipes, a computer can generate a signal to drive the speaker and cancel out the exhaust noise. Active noise-canceling mufflers are not being used on automobiles just yet. In the future, perhaps.
Source: BMW of Rockville