Grey noise is random noise whose
frequency spectrum
The power spectrum S_(f) of a time series x(t) describes the distribution of power into frequency components composing that signal. According to Fourier analysis, any physical signal can be decomposed into a number of discrete frequencies, ...
follows an
equal-loudness contour (such as an ''inverted''
A-weighting curve).
The result is that grey noise contains all frequencies with equal
loudness
In acoustics, loudness is the subjective perception of sound pressure. More formally, it is defined as, "That attribute of auditory sensation in terms of which sounds can be ordered on a scale extending from quiet to loud". The relation of ph ...
, as opposed to
white noise
In signal processing, white noise is a random signal having equal intensity at different frequencies, giving it a constant power spectral density. The term is used, with this or similar meanings, in many scientific and technical disciplines ...
, which contains all frequencies with equal ''energy''. The difference between the two is the result of
psychoacoustics
Psychoacoustics is the branch of psychophysics involving the scientific study of sound perception and audiology—how humans perceive various sounds. More specifically, it is the branch of science studying the psychological responses associated wi ...
, more specifically the fact that the human hearing is more sensitive to some frequencies than others.
Since equal-loudness curves depend not only on the individual but also on the volume at which the noise is played back, there is no one true grey noise. A mathematically simpler and clearly defined approximation of an equal-loudness noise is
pink noise
Pink noise or noise is a signal or process with a frequency spectrum such that the power spectral density (power per frequency interval) is inversely proportional to the frequency of the signal. In pink noise, each octave interval (halving ...
which creates an equal amount of energy per octave, not per hertz (i.e. a logarithmic instead of a linear behavior), so pink noise is closer to "equally loud at all frequencies" than white noise is.
See also
*
Colors of noise
In audio engineering, electronics, physics, and many other fields, the color of noise or noise spectrum refers to the power spectrum of a noise signal (a signal produced by a stochastic process). Different colors of noise have significantly ...
References
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Noise (electronics)