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Exhaust gases leave the engine under extremely high pressure. If
these gases escaped directly from the engine the noise would be
tremendous. For this reason, the exhaust manifold sends the gases to
a muffler where they go through metal plates, or tubes, with a series
of holes. The pressure of the gases is reduced when they pass through
the muffler, so they go out of the tail pipe quietly.
The muffler is made of metal and is located underneath the body of
the car. It's connected between the tail pipe and the catalytic
converter.
There are two types of muffler design. One type uses several baffled
chambers to reduce noise. The other type sends the gases straight
through perforated pipe wrapped in metal or fiberglass. This type of
muffler is designed for the purpose of reducing back pressure and,
consequently, makes slightly more noise.
Since a muffler cannot reduce the noise of the engine by itself, some
exhaust systems also have a resonator. Resonators are like little
mufflers, and are usually the "straight through" type. They are added
at the end of the exhaust system to take care of any noise that has
made it through the muffler.
The muffler quiets the noise of the exhaust by "muffling" the sound
waves created by the opening and closing of the exhaust valves. When
an exhaust valve opens, it discharges the burned gases at high
pressures into the exhaust pipe, which is at low pressure. This type
of action creates sound waves that travel through the flowing gas,
moving much faster than the gas itself (up to 1400 m.p.h.), that the
muffler must silence. It generally does this by converting the sound
wave energy into heat by passing the exhaust gas and its accompanying
wave pattern, through perforated chambers of varied sizes. Passing
into the perforations and reflectors within the chamber forces the
sound waves to dissipate their energy.
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