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How Mufflers Work

In the previous post we highlighted how the sound is generated due to pressure waves.

A muffler is also known as a silencer. In our Kenyan market a catalyst/ catalytic converter is also referred to as a muffler.in this post we shall refer the muffler as the silencer and not as the catalytic converter.

The key thing about sound waves is that the result at your ear is the sum of all the sound waves hitting your ear at that time. If you are listening to a band, even though you may hear several distinct sources of sound, the pressure waves hitting your ear drum all add together, so your ear drum only feels one pressure at any given moment.

However by producing a sound wave that is exactly the opposite of another sound wave a constructive or destructive interference can be achieved.

If the two waves are in phase, they add up to a wave with the same frequency but twice the amplitude leading to constructive interferance

But, if they are exactly out of phase, they add up to zero leading to destructiveinterference. In this case at the time when the first wave is at its maximum pressure, the second wave is at its minimum. If both of these waves hit your ear drum at the same time, you would not hear anything because the two waves always add up to zero.

Using this concept, automotive engineers design mufflers which make the cars and trucks as they sound.

Basically, muffler is a set hollow tubes enclosed in a jacket and sealed off at the ends.

These tubes are designed to create reflected waves that interfere with each other or cancel each other out.

The exhaust gases and the sound waves enter through the centre/ main tube. They bounce off the back wall of the muffler casing and are reflected through a hole into the main body of the muffler. They pass through a set of holes into another chamber, where they turn and go out the last pipe and leave the 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.

In reality, the sound coming from the engine is a mixture of many different frequencies of sound, and since many of those frequencies depend on the engine speed, the sound is almost never at exactly the right frequency for this to happen. The resonator is designed to work best in the frequency range where the engine makes the most noise; but even if the frequency is not exactly what the resonator was tuned for, it will still produce some destructive interference.

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, cancelling each other out to some extent in addition to being absorbed by the muffler’s housing.

Backpressure and Other Types of Mufflers

The exhaust system for race cars does not have mufflers because reducing backpressure is the ultimate design focus

One important characteristic of mufflers is how much backpressure they produce. Because of all of the turns and holes the exhaust gas has to go through, a fairly high backpressure is produced. This back pressure 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.

These mufflers produce much less restriction, but don’t reduce the sound level as much as conventional mufflers.

Active Noise-Cancelling Mufflers

There have been a few experiments with active noise-cancelling 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.

With all the automotive producers trying to achieve minimal emissions and sound pollution, the exhaust system is also making to the list of computerized car components.