In science, and especially in physics and telecommunication, noise is fluctuations in and the addition of external factors to the stream of target information (signal) being received at a detector. In communications, it may be deliberate as for instance jamming of a radio or TV signal, but in most cases it is assumed to be merely undesired interference with intended operations. Natural and deliberate noise sources can provide both or either of random interference or patterned interference. Only the latter can be cancelled effectively.
More specifically, in physics, the term noise has the following meanings:
An undesired disturbance within the frequencyband of interest; the summation of unwanted or disturbing energy introduced into a communications system from man-made and natural sources.
A disturbance that affects a signal and that may distort the information carried by the signal.
Random variations of one or more characteristics of any entity such as voltage, current, or data.
A random signal of known statistical properties of amplitude, distribution, and spectral density.
Loosely, any disturbance tending to interfere with the normal operation of a device or system.
Noise and what can be done about it has long been studied. It was Shannon who established information theory and in so doing clarified the essential nature of noise and the limits it places on operation of our (or anyone's) equipment. Shannon's work was a breakthrough.
In some cases a little noise may be considered advantageous, allowing a dithered representation of signals below the minimum strength, or between two quantization levels. This is especially true for signals intended for human appreciation, since the brain seems to expect signals to contain a degree of noise. See, for example [1] (http://www.oreilly.com/catalog/mindhks/chapter/hack33.pdf)
Noise is a recognized form of pollution, but is difficult to measure because the annoyance or discomfort caused varies between individuals.
Loud noise is a major pollutant in towns and cities.
Electronic noise takes the form of unwanted signals generated in electronic circuits and in recording processes by stray electrical or magnetic fields, or by temperature variations.
Noise can improve human balance control, to the point that it may enable elderly subjects to steady themselves as well as their young counterparts, researchers in New England have demonstrated (Jim Collins, Boston University, 617-353-0390, jcollins@bu.edu).
The authors further demonstrate a trend in elderly subjects towards reducing their postural sway to the level of young subjects, suggesting that noise may be a "fountain of youth" for human balance.
Noise may provide similar beneficial effects in individuals with marked sensory deficits, such as patients who have suffered a stroke or a disorder in the peripheral nervous system.
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