What is red noise? Red noise vs brown noise, explained

Red noise is another name for brown noise: random sound whose power falls about 6 decibels per octave, so it rumbles like distant surf rather than hissing like static. Slo plays it free in the browser at slonoise.com under the brown noise name, and the same page lets you switch between it, pink noise and white noise.

Red noise vs brown noise

In audio they are the same signal. Both names describe noise whose power at a given frequency falls with the square of that frequency, written 1/f². Each octave you climb holds about 6 decibels less power than the one below it. Double the frequency and the power drops to a quarter.

The two names come from different places. The name brown does not come from the colour. It comes from Brownian motion, the jittery drift the botanist Robert Brown described in 1827 in particles from pollen grains suspended in water. Add up white noise sample by sample and you get that kind of random walk, and its spectrum follows the 1/f² slope. Red borrows the light analogy that named white and pink noise: red light sits at the low-frequency end of the visible spectrum, and this noise piles its power at the low end of the audible one.

Neither name has a standard behind it. The ASA/ANSI acoustical terminology database defines white noise and pink noise, but its entries for brown noise and red noise return 404, checked on 10 September 2026. When an app lists red and brown noise as two different sounds, nothing official backs the split, so ask what slope each one uses.

What scientists mean by red noise

Outside audio the word stretches. Climate scientists and others who analyse time series use red noise for a signal where each value leans on the one before it, a lag-1 autoregressive or AR(1) process. Its spectrum stays flat at the lowest frequencies and then falls away. Torrence and Compo’s widely used guide to wavelet analysis takes that model as the background a real climate signal has to stand out from.

So red noise in a research paper may not mean a clean 6 decibels per octave. In a sleep or focus app it almost always does.

How Slo’s brown noise measures

Slo’s generator happens to be red noise in the scientific sense too. The code in slo-noise-worklet.js runs a leaky integrator over white noise: each new sample keeps about 98% of the previous one and adds a small slice of fresh randomness. That is an AR(1) process.

We rendered the same formula for 30 seconds at 48 kHz on 13 September 2026 and averaged its spectrum with 8192-point segments. Between 400 Hz and 6.4 kHz the level falls 5 to 6 decibels per octave, the brown slope. Below about 150 Hz it levels off instead of climbing without limit, which stops the deepest bass from swamping a small speaker.

The site’s own measurements from 4 September 2026 show where the power lands:

Sound	Slope	Power below 250 Hz	Power above 4 kHz	Mean frequency, Hz
Brown (red) noise	Falls about 6 dB per octave	65.9%	2.3%	537
Pink noise	Falls about 3 dB per octave	52.5%	18.7%	2,511
White noise	Flat	1.0%	83.4%	12,026

These figures describe Slo’s generators. Another app’s red noise may use a steeper or gentler filter, and your speakers and headphones reshape all of it before it reaches you.

What red noise sounds like

A deep, soft roar with almost no hiss. Listeners compare it to a waterfall heard from a distance or the inside of a plane at cruise. With only 2.3% of its power above 4 kHz, it covers low sounds such as traffic rumble or a neighbour’s bass better than it covers voices, whose consonants sit higher. For speech through a wall, white noise or pink noise reaches further up.

Is red noise good for sleep?

No study has tested red or brown noise on its own for sleep. The World Health Organization’s 2018 guidelines establish that environmental noise can disturb sleep, and that dealing with the source beats covering it where you can. Riedy and colleagues reviewed the human studies of continuous noise in 2021 and rated the evidence very low quality with mixed results.

Treat red noise as a preference. Some people find a steady rumble easier to ignore than a quiet room broken by sudden sounds; others find it one more thing to hear. Set the volume to the lowest level that does the job. The WHO advises adults to keep average personal-audio exposure below 80 dB, and to listen for less time as the level rises.

Questions people ask

Is red noise the same as brown noise?

In audio, yes. Both names describe noise whose power falls about 6 decibels per octave. Brown comes from Brownian motion and red from the light analogy, and no standards body defines either one.

What does red noise sound like?

A deep, soft roar with almost no hiss, close to a distant waterfall or the cabin of a plane at cruise. It covers low sounds such as traffic rumble better than it covers voices.

Is red noise good for sleep?

No study has tested it on its own. A 2021 systematic review of continuous noise and sleep rated the evidence very low quality with mixed results, so treat red noise as a preference and keep the volume low.

What does red noise mean in climate science?

A signal where each value leans on the one before it, called a lag-1 autoregressive process. Its spectrum is flat at the lowest frequencies and falls away above them, so it need not follow a clean 6 dB per octave slope.

Read next

• Brown noise vs white noise compares the rumble with the hiss.
• What is green noise covers the mid-range colour with no standard definition.
• Noise colours explained runs through blue, violet and grey as well.
• Brown noise for sleep opens the bedtime preset with a timer.

Sources

• White noise, entry 2.37 and pink noise, entry 2.38, ASA/ANSI Standard Acoustical and Bioacoustical Terminology database. Brown noise and red noise return 404.
• Torrence C, Compo GP (1998), A Practical Guide to Wavelet Analysis, Bulletin of the American Meteorological Society 79(1).
• World Health Organization (2018), Environmental Noise Guidelines for the European Region.
• Riedy and colleagues (2021), Noise as a sleep aid: A systematic review.
• World Health Organization, Deafness and hearing loss: Safe listening.
• First-party: a 30-second render of the brown-noise formula in public/audio/slo-noise-worklet.js, 13 September 2026, and band shares in src/data/sound-measurements.ts, measured 4 September 2026.

Filip Kowalski is the developer of Slo and writes about sound, sleep, and focus.