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Noise cancellation is a method to reduce
or completely cancel out undesirable sound, such that
you can't hear it. It is often call Active Noise
Cancellation because the electronics involved actively
cause the noise reduction in real time. The most common
electronic noise reduction or sound suppression device
consists of special earphones. Other types of devices
are being developed.
Questions you may have about such a
device are:
Noise relief needed
There are many situations where people
are working in an environment that has high noise that
cannot be suppressed. In many cases, you see workers
wearing headsets or earplugs to help block out some of
the noise.
For example, airport workers often wear
rigid cup headsets to reduce the noise to their ears.
Some factory workers wear earplugs, as do members of
heavy metal rock bands. This is done to protect their
hearing from damage due to excessive noise levels.
Even students would like to have relief
from noisy surroundings when they are trying to
concentrate on their studies. Noisy surroundings can
even make it difficult to listen to music with a
headset.
That is why some sort of noise
suppression device would be handy.
How noise cancellation works
Noise is just sound that is too loud and
not useful to you. It could be the roar of engines,
clanking of machinery, or the load conversation of rude
people. Noise is something you don't care to listen to.
All sound consists of a waveform. Simple
illustrations are the waveform of a vibrating guitar
string or the waves in a pool of water.
If you add two waves together that are
going in the same direction, and if those waves are in
phase--that is, the peaks and valleys of the waves line
up--then the amplitude or height of the waves will
double. That would be like two sources of the same sound
would double the volume.
Now, if you add two waves together that
are going in the same direction, and if those waves are
completely out of phase--that is, the peaks of one line
up with the valleys of the other--then the amplitude or
height of the waves cancel each other out.
Waves out of phase will cancel each
other
In the illustration above, the addition
of the two waves will result in a flat wave or line in
the middle. In this special situation, you would hear no
sound.
Canceling out sound waves can be done
electronically. There are now special noise suppression
headsets that have a microphone and electronics built
in. The microphone detects the noise, changes it to an
electrical signal and relays it to the speaker in the
headset, which turns the signal back into sound. This is
how any microphone-speaker system works.
What makes it different is that the
electronics puts the recorded signal exactly out of
phase with the actual sound, which is so loud that it
easily gets through the headset to the person's ear. The
sound from the headset then is just the same sound and
as loud as the noise, but it is completely out of phase
with the noise, thus canceling the sound.
What is amazing here is that we can add
two loud noises that are simply out of phase, and we end
up with silence!
Applications
There are a number of great applications
for noise suppression or cancellation devices.
One obvious application is that people
working near aircraft or in noisy factories can now wear
these electronic noise cancellation headsets to protect
their hearing.
Students can wear these headsets to
cancel out unwanted noise, while being able to listen to
their own music while they study.
Honda is now using noise-cancellation
technology in their Japan-only Accord station wagon.
The way it works is that a microphone
connected to the car stereo system picks up all the
sound inside the car, including music or such from the
stereo. Then the noise-cancellation system subtracts the
sound of the music coming from the stereo and produces
noise-canceling sound waves that match the frequency of
unwanted sound.
The noise-canceling sound waves are also
sent through the stereo speakers, along with the music.
This technique greatly reduces the low frequency
vibration noises in the car, without dampening the car's
audio system.
Unfortunately, as of November 2000,
Honda is not shipping the noise-cancellation system
outside Japan.
A piezoelectric device creates an
electrical signal when it detects a vibration. It also
will vibrate according to an electrical signal it
receives.
Some space satellites have long
antennas. If such an antenna would start to vibrate
wildly, it could through the satellite out of orbit and
out of control. By detecting the waveform or any
vibration in the antenna, it can be suppressed in the
same way that noise is suppressed.
Putting piezoelectric devices on the
antenna can result in vibrating the antenna in an
opposite phase, thus eliminating the dangerous motion.
One cool idea for piezoelectric devices
is to suppress unwanted noise from another apartment.
For example, suppose the person in the
next apartment has his stereo on real loud. In some
cases, you can even feel the wall vibrate. By placing
piezoelectric devices on the wall, they can detect the
wall vibrations and generate vibrations in an opposite
phase. Thus would completely cancel out the noise coming
from the other apartment.
Unfortunately, such a configuration is
too expensive for the average person. I don't think
there is anything commercially available. But it is a
good idea and application.
Noise can be suppressed or cancelled by
electronically creating the same waveform and volume,
but in an opposite phase. The combinations of waves
cancel each other out, resulting in near silence.
Noise cancellation devices can protect
the ears of workers, as well as to allow people to
concentrate on what they want to do. |
