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White noise is a signal (or process) with a flat frequency spectrum. In other words, the signal has equal power in
a given bandwidth at any centre frequency.
Therefore a White noise signal is purely a theoretical construct. By having power at all frequencies, the
total power of such a signal would be infinite. In practice a signal can be White with a flat spectrum over a
defined frequency band.
Defining a signal as White in the frequency domain defines important statistical properties of the signal in
time. That is, a signal which is statistically uncorrelated (see Correlation)
with itself over time is by definition white.
Uncorrelated in time does not however restrict the values a signal can take. Any distribution of values is possible. For
example, a binary signal which can only take on the values 0 or 1 will be White if the sequence of zeros and
ones is statistically uncorrelated.
It is often incorrectly assumed that Gaussian noise (see normal
distribution) is White Noise. The two properties are not related. However, Gaussian White
Noise is often specified; such a signal has the useful statistical property that it's values are independent in
time.
In digital signal processing, almost all types
of noise are often approximated with white noise because of the useful statistical properties. In practical applications few
signals are exactly white noise, or even close to white noise. However, signals such as gunshot noise and thermal noise have a
close to flat spectrum.
A closely related concept is pink noise which is a sound that is
perceptually white noise. That is, the human auditory system perceives equal magnitude on all frequencies.
One use for white noise is in the field of architectural acoustics. Here in order to submerge distracting, undesirable noises (for example
conversations, etc.,) in interior spaces, a constant level of sound at a desired decibel level is generated and provided as a background. In this context it is also referred to as grey
noise.
White noise has also been used in much electronic music.
See also: Statistics
White Noise is a 1985 novel by Don DeLillo.
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