Gray Noise

You are lying on your back, eyes closed, and your muscles are deeply relaxed into the bed. You are listening intently to the barely audible hum of a motor nearby. It moves the fresh air coming in from the open window with a rhythm of its own. The breeze it creates settles softly on your body and eases you into sleep.

Grey noise is random noise subjected to a psychoacoustic equal loudness curve (such as an inverted A-weighting 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, noise which is in fact equally loud at all frequencies but not perceived as such due to psychoacoustics.