Noise-canceling technology is based on the fact that a sound is a wave
with a particular shape. An external noise-canceling speaker cancels a
sound wave by producing an opposite wave to the sound, dampening the
original sound and producing something closer to silence.
Acquiring a Feed
To produce the opposite sound wave from a sound that is being emitted in a
room, the external noise-canceling speaker begins by acquiring a feed of
the sounds that need to be canceled. The external noise-canceling speaker
acquires the sound feed by recording it through an external microphone. In
some setups, this microphone is on the speaker itself, and in other
setups, the microphone acquiring the sound feed is a distance away from
the speaker. Because of a limitation on power, as well as the changes that
occur in a sound wave depending on the listener's or microphone's
location, sound-canceling technology is only effective in a small area,
such as in a room or patio enclosure.
Once an audio feed is acquired through a microphone, it is processed to
determine the best wave to use to cancel the original sound. Sounds that
are recurring and static in volume, such as a driving noise, a motor noise
or static on a radio, are more likely to be successfully canceled by a
noise-cancellation speaker. Sounds that are spontaneous or have rapid
changes in volume are more difficult for the sound-canceling technology to
react to, making them less likely to be successfully canceled.
Manufacturers of sound-canceling technology have discussed options that
allow early detection of a faraway sound to give the processor more time
to cancel an approaching spontaneous or rapidly increasing sound over a
larger area. This technology would be helpful for neighborhoods with noise
problems from passing trains, or other loud and sudden noises.
After the ideal cancellation wave is determined by the noise-cancellation
technology, the wave is fed back through an external speaker and should
have a significant dampening effect on the original sound. This technology
is useful in areas with a lot of noise, such as those near an airport or
concert venue. The same technology is used on a smaller scale in
Reprint from eHow.com, by Terry Morgan