White noise has long been used as a sleep and relaxation aid. But what is white noise, and could there be other "colors" of noise that are even more effective for relaxation and even memory enhancement?
White light has all the colors of the rainbow equally represented, which is why it doesn't look like any one particular color. Likewise, the frequencies in white noise are equally represented, with the lowest audible frequency having the same power as all the frequencies above it. Because white noise contains all frequencies, it can help mask other noises like the ringing of tinnitus... but humans don't hear all frequencies equally. The distance between 30 and 60 hertz sounds the same as the distance between 10,000 and 20,000 hertz, and we're more sensitive to high pitched noise.
Enter pink noise. The energy in pink noise is highest for low frequencies and is halved every time the frequency doubles, meaning every octave has equal power and the net effect sounds less bright and more balance than white noise. While white noise is by far the most researched white color, pink noise studies are on the rise recently. One study from 2012 found that participants who listened to pink noise while they slept showed an improvement in deep sleep and reported sleeping better.
But pink isn't the only other "color" of noise out there... blue noise, brown noise, and gray noise all have official or colloquial definitions, and scientists are just beginning to understand the complex connections between sound and cognitive function.