If you've ever heard a car engine
running without a muffler, you know what a huge difference a
muffler can make to the noise level. Inside a muffler, you'll
find a deceptively simple set of tubes with some holes in
them. These tubes and chambers are actually as finely tuned as
a musical instrument. They are designed to reflect the sound
waves produced by the engine in such a way that they partially
cancel themselves out.
Mufflers use some pretty neat technology to cancel out the
noise. In this edition of HowStuffWorks,
we'll take a look inside a real car muffler and learn about
the principles that make it work.
But first, we need to know a little about sound.
Where Does the Sound Come From? Sound is a
pressure wave formed from pulses of alternating high
and low air pressure. These pulses makes their way through the
air at -- you guessed it -- the speed of
sound.
In an engine,
pulses are created when an exhaust valve opens and a burst of
high-pressure gas suddenly enters the exhaust system. The
molecules in this gas collide with the lower-pressure
molecules in the pipe, causing them to stack up 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 much faster than
the actual gases do.
When these pressure pulses reach your ear,
the eardrum vibrates back and forth. Your brain
interprets this motion as sound. Two main characteristics of
the wave determine how we perceive the sound:
Sound wave frequency - A higher wave frequency
simply means that the air pressure fluctuates faster. The
faster an engine runs, the higher the pitch we hear. Slower
fluctuations sound like a lower pitch.
Air pressure level - The wave's amplitude
determines how loud the sound is. Sound waves with greater
amplitudes move our eardrums more, and we register this
sensation as a higher volume.
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? 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.
Now comes the cool part: It is possible to produce a sound
wave that is exactly the opposite of another wave. This is the
basis for those noise-canceling headphones you may have seen.
Take a look at the figure below. The wave on top and the
second wave are both pure tones. If the two 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.
How sound waves add and
subtract
In the next section, we'll see how the muffler is designed
to create waves that cause as much destructive interference as
possible.
Inside a Muffler Located inside the muffler
is a set of tubes. These tubes are designed to create
reflected waves that interfere with each other or cancel each
other out. Take a look at the inside of this muffler:
The exhaust gases and the sound waves enter through the
center tube. They bounce off the back wall of the muffler 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? 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.
Waves canceling inside a simplified
muffler
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, canceling each other out to some extent in addition
to being absorbed by the muffler's housing.
Backpressure and Other Types of Mufflers 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.
The exhaust from a NASCAR
race car: There are no mufflers here, because
reducing backpressure is the name of the
game.
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
These mufflers produce much less restriction, but don't
reduce the sound level as much as conventional mufflers.
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.
