White, pink or brown noise?

White, pink and brown noise get mentioned together all the time, usually with little explanation. Here is what actually separates them, in plain words first and numbers second, how each one sounds, and how to pick between them.

What each one is

All three are noise in the sound engineer's sense: a random, steady sound with every pitch in it at once, and no tune, beat or pattern to follow. The colour names describe one thing only, the balance between low pitches and high ones. They're borrowed from light. White light was once assumed to hold every colour in equal measure, so noise holding every frequency in equal measure was called white.

White noise puts the same energy into every frequency. That sounds perfectly even, but it isn't how we hear. We hear pitch in octaves, and an octave is a doubling: from the A below middle C, at 220 Hz, up to the A above it, at 440 Hz, is one octave. The octaves get wider as they climb. The top octave of hearing runs from about 10,000 to 20,000 Hz, a band 10,000 Hz wide, while the bottom one, from 20 to 40 Hz, is only 20 Hz wide. Give every frequency the same energy and the top octave alone ends up with half of it, which is why white noise sounds so bright.

Pink noise gives every octave the same energy instead. To manage that, it has to get quieter as the pitch climbs: about 3 decibels (dB) quieter per octave, which halves the power at each step up. Each octave up covers twice as many frequencies, each at half the power, so every octave carries the same total. Audio engineers often use pink noise as a reference signal.

Brown noise falls twice as steeply, by about 6 dB per octave, so its power drops to a quarter with each octave up. Most of its energy sits in the bass, with only a thin layer of treble on top. Despite the name, it has nothing to do with the colour. It's named after Brownian motion, the random jostling of tiny particles in a liquid or gas, because a brown noise signal wanders in that same random way. You'll also see it called red noise.

What each one sounds like

Numbers aside, the three are easy to tell apart by ear, and everyday sounds make good reference points. Real sounds are never exactly white, pink or brown, but each of these is close enough to help.

• White is a bright, even hiss: a radio between stations, a tyre losing air, a burst of steam. It's crisp and airy, and close up or played loud it can turn harsh.
• Pink is softer and rounder, with the hiss pulled back and more body underneath. People liken it to steady rain, wind moving through trees or a far-off waterfall. The Sleep Foundation's article on pink noise lists rain, waterfalls, wind, rivers and ocean waves among the sounds it's often compared to.
• Brown is deep and muffled, a low roar with its edges rounded off: surf breaking a little way down the beach, a big river in full flow, or a storm heard from indoors.

You can hear all three on the fan above. Listen for the hiss at the top of the sound: it's strongest on White, gentler on Pink, and almost gone on Brown, while the low whoosh grows from one to the next.

How to choose

There's no correct answer here, and no colour is better than the others. It comes down to the sound you like and, sometimes, to the sound you'd like to cover.

• Start with what you enjoy. If a crisp, airy sound appeals, try white. If you want something that simply sits there evenly, try pink. If you like a deep, soft rumble, try brown.
• Think about what's around you. A steady sound tends to blend best with noise in the same range. Voices through a wall or the clatter of a kitchen fold more easily into white or pink; an engine idling outside or a washing machine on its spin cycle folds more easily into brown.
• If one sounds wrong, move one step. White too hissy? Try pink. Brown too muffled? Try pink as well. It's the middle ground in both directions.
• Give each one a few minutes, at the volume you'd really use. A sound can seem quite different turned down low, and a first impression of a noise doesn't always last. The one you stop noticing soonest may be the one to keep.

You'll find plenty of claims about these noises online, and researchers have studied pink noise and sleep in particular. This guide sticks to what each noise is and how it sounds. The Sleep Foundation's article, listed under Sources, gives an overview of that research and notes that more studies are needed.

How Fanno uses them

Many noise apps offer white, pink and brown as three separate sounds. Fanno uses them as the tone of one fan. The White, Pink and Brown keys on its deck change the character of the air the fan moves, and everything else about the fan stays as it is. You choose the fan first, a box fan, desk fan, ceiling fan or air purifier, and then how bright or deep its air should be.

So Fanno's White isn't white noise straight out of a lab. It's the fan turned bright: the full rush of its air, with the sharpest top smoothed away. Pink rolls more of the top off and brings the low whoosh up a little. Brown rolls it off much further and lets the low end lead. All three still sound like a fan rather than a piece of test equipment.

The cards above switch the same box fan between the three while it plays, so you can compare them directly. In the app, every fan can play every tone, and the dial's ten speeds change each of them again: low speeds soften any tone, and high ones open it up into a fuller rush. Because Fanno makes its sound live, none of them ever loops. For more on each tone, see the pages on white noise, pink noise and brown noise.