Noise Canceling 

Noise canceling technology is a narrow field of noise control that, in theory, reads the intruding noise and then sends a “canceling” or ”damping” sound wave toward the sound source. The technology is very difficult to implement in a real world three dimensional space. Some fan exhaust systems have been successful using this technology but has limited success in most industrial and outdoor applications.

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Scientific Basis
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.
 
Processing
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.
 
Playback
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 noise-canceling headphones

                                                                                    Reprint from eHow.com, by Terry Morgan