Colors of noise |
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**Grey noise** is random noise whose frequency spectrum follows an equal-loudness contour (such as an *inverted* A-weighting curve).

The result is that grey noise contains all frequencies with equal loudness, as opposed to white noise, which contains all frequencies with equal *energy*. The difference between the two is the result of psychoacoustics, 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.^{[1]} A mathematically simpler and clearly defined approximation of an equal-loudness noise is pink noise 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.^{[2]}