car

[ginettev]

how does a car silencer work?

[Mortartube]

Basically by slowing down the exhaust gasses. The exhaust gasses pass down the narrow exhaust pipe and then go into a much bigger chamber (The silencer), this slows down the gasses and allows the gas pressure to drop causing a silencing effect. Imagine two balloons filled with identical amounts of air. If you pop one with a pin it makes a bang as a large amount of air is released quickly. If you untie the other balloon the same amount of air is released but more slowly. The car silencer uses this principle.

[Clanad]

While there are aspects of mortartube's explanation that I would agree with, the full answer is more complicated.    Firstly, in the U.S., the device is called a muffler. 

Background: If two sound waves are in phase, they add up to a wave with the same frequency but twice the amplitude. This is called constructive interference. But, if they are exactly out of phase, they add up to zero. This is called destructive interference. 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.

Located inside the muffler is a set of tubes inside chambers. These tubes are designed to create reflected waves that interfere with each other or cancel each other out.

Another chamber, called a resonator, inside the muffler 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. 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.

Contd.

[Clanad]

Contd.

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.

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 (silencer?) housing...

(With partial thanks to How Stuff Works)

[landie]

My god, I've wasted my money. I've just splashed out on a new one and couldn't get my head around the fact that I could still hear the missus.

Exhaust gasses eh, who'd have thought it?