White, pink and brown noise: what the colours mean

The colour of noise describes how its energy is spread across frequencies. White noise has equal energy at every frequency and sounds like a bright hiss. Pink noise has less energy at high frequencies and sounds softer and more balanced, like steady rain. Brown noise falls away even faster and sounds like a deep rumble or distant surf. None is better in general; which one you prefer is mostly a matter of taste. The noise generator makes all three as downloadable files, so you can compare them side by side.

White noise

White noise is random sound with the same average power in every frequency band of equal width: the band from 100 to 200 Hz holds as much energy as the band from 10,000 to 10,100 Hz. Because the higher octaves contain far more hertz than the lower ones, most of white noise's energy sits in the treble, which is why it sounds hissy, like an untuned radio or escaping steam. The name is an analogy with white light, which mixes all visible frequencies. In audio work it is used to test equipment and as raw material for synthesising sounds. The generator makes it the simplest possible way, by giving every sample an independent random value.

Pink noise

Pink noise has equal energy in every octave: the band from 100 to 200 Hz holds the same energy as 1,000 to 2,000 Hz. Its power falls by about 3 decibels each time the frequency doubles. Human hearing judges pitch in octaves, so pink noise tends to sound even from bass to treble rather than tilted toward the top, and that evenness is why it is the standard test signal for setting up speakers and tuning rooms. Play both into the spectrum analyzer and you can see the slope: pink noise's level falls steadily toward high frequencies compared with white. The generator builds pink noise by adding together several random sources that update at octave-spaced rates, a method known as the Voss-McCartney algorithm.

Brown noise

Brown noise, also called red or Brownian noise, falls by about 6 decibels per octave, so most of its energy is in the bass. It is named after Brownian motion, the random jiggling of tiny particles described by the botanist Robert Brown, not after the colour: it can be made by keeping a running total of random steps, a random walk. To the ear it is a low, soft roar, closer to a waterfall, heavy rain on a roof or the cabin of an aircraft. Many people find it the most relaxing of the three, and it is the default in the white noise player. The generator makes it as a slightly leaky random walk, so the signal cannot drift away from zero.

Blue, violet and grey

The colours continue. Blue noise rises by about 3 dB per octave and violet by about 6 dB, so both sound sharp and are used more in signal processing, such as dithering images, than for listening. Grey noise is shaped to follow the ear's equal-loudness curves, so it sounds roughly equally loud at all frequencies to a typical listener. Other names, such as green noise, are used loosely and have no single agreed definition.

Sleep, focus and masking

Steady noise works mainly by masking. A constant background sound makes sudden ones, such as traffic, a snoring partner or a neighbour's door, less noticeable, because the change in sound is smaller. Many people find this helps them fall asleep or concentrate, and some people simply prefer silence. Research on noise for sleep is limited and of mixed quality: some small studies report benefits and others find none, and claims that a particular colour boosts deep sleep or focus are not well established. If you want to try it, test each colour at a low volume for a few nights and keep what you like. Noise is also used in sound therapy for tinnitus, usually with guidance from an audiologist. If you are curious how noise compares with tones, Do binaural beats work? The illusion and the evidence looks at that evidence, and Sleep cycles, and waking up less awful covers how a night's sleep is structured.

Listening safely

Because noise plays for hours, volume matters more than colour. Keep it only as loud as it needs to be to cover the sounds you want to hide: a comfortable background level, not loud. For babies, place any sound machine well away from the cot and keep it quiet. With headphones, use a timer or a lower volume, and if you notice ringing in your ears or muffled hearing afterwards, turn it down. The hearing test gives a rough check of the highest frequency you can hear, though it is no substitute for a professional test. The noise generator exports 44.1 kHz mono WAV files, about 25 MB for five minutes, that loop seamlessly, so you can play them on any device without streaming.