Grey noise is random noise subjected to a psychoacoustic equal loudness curve (such as an inverted A-weighting

A-weighting

A Weighting curve is a graph of a set of factors, that are used to 'weight' measured values of a variable according to their importance in relation to some outcome. The most commonly known example is frequency weighting in sound level measurement where a specific set of weighting curves known as A,...

 curve) over a given range of frequencies, giving the listener the perception that it is equally loud at all frequencies.

This is in contrast to white noise

White noise

White noise is a random signal with a flat power spectral density. In other words, the signal contains equal power within a fixed bandwidth at any center frequency...

, noise which is in fact equally loud at all frequencies but not perceived as such due to psychoacoustics

Psychoacoustics

Psychoacoustics is the scientific study of sound perception. More specifically, it is the branch of science studying the psychological and physiological responses associated with sound...

. See Colors of noise

Colors of noise

While noise is by definition derived from a random signal, it can have different characteristic statistical properties corresponding to different mappings from a source of randomness to the concrete noise. Spectral density is such a property, which can be used to distinguish different types of noise...

.