Green noise the middle of the spectrum

What is green noise?

Green noise is an informal name for noise whose energy is concentrated in the middle of the audible range, most often described as pink-like noise with more energy around 500 Hz. Unlike white, pink, brown, blue and violet noise, it has no standard definition: there is no agreed formula, slope or bandwidth, and two recordings sold as green noise can sound quite different.

What they have in common is the shape. The standard colours are defined by a slope, a steady tilt of the spectrum up or down. Green noise is defined by a hump: less energy at both ends and more in the middle. That is why it sounds neither hissy like white noise nor rumbling like brown noise, but rounded and soft.

The colour name follows the same loose analogy with light as the other noise colours. Green sits in the middle of the visible spectrum, so a noise with its energy in the middle of the audible spectrum was called green. It is a metaphor, not a measurement.

Where does the 500 Hz idea come from?

The 500 Hz description of green noise goes back to an informal online text rather than to a standard. Joseph S. Wisniewski's Colors of noise pseudo FAQ, posted to the comp.dsp newsgroup for digital signal processing in 1996 and cited by the Wikipedia article on the colours of noise, describes green noise as the background noise of the world: a spectrum similar to pink noise with more energy around 500 Hz.

The official glossaries do not go that far. Federal Standard 1037C, the US telecommunications glossary that defines white, pink, blue and black noise, has no entry for green noise. So the 500 Hz figure is a convention that spread through sound apps and ambient recordings, not a value set by any acoustics body.

That history is worth knowing because it explains the variety you will hear. Some green noise tracks are pink noise with a gentle bump near 500 Hz; others are narrow bands that sound almost like a hum. Both are called green, and neither is more correct than the other.

Green noise in image processing and printing

In image processing green noise means something entirely different: a pattern of dots dominated by middle spatial frequencies, used in halftone printing. Halftoning is how a printer renders shades of grey with dots of a single ink; the arrangement of those dots can be described by its frequency content just like a sound.

The term was set out by D. L. Lau, G. R. Arce and N. C. Gallagher in Green-noise digital halftoning, Proceedings of the IEEE (1998). There it has a precise technical meaning: dot patterns whose structure is dominated by middle frequencies, neither fine and even nor coarse and blotchy.

This is the only field in which green noise has a formal definition, and it has nothing to do with sleep sounds. If you search for green noise and find papers about printing, that is why.

What does green noise sound like?

Green noise sounds like a soft, steady rush with a rounded middle: less bright than white noise, less deep than brown, closer to pink but with the edges trimmed. People compare it to a waterfall heard from some distance or to wind in a forest.

The version in our generator is built in three steps from pink noise. A high-pass filter reduces everything below about 120 Hz, so there is little rumble. A low-pass filter reduces everything above about 2,200 Hz, so there is little hiss. A peak filter adds about 7 dB centred on 500 Hz, which gives the sound its full middle. The result is what most descriptions of green noise mean, stated in numbers instead of adjectives.

Because the top and bottom are trimmed, green noise can feel quieter than white or pink noise at the same volume setting. Set the level by ear, not by the position of the slider.

Green noise vs pink, brown and white noise

Green noise differs from the standard colours by having a peak in the middle instead of a steady slope. The table places it among them.

The masking row follows from how masking works: a steady sound hides other sounds most strongly when they share frequencies. Green noise has little energy in the bass and in the treble, so it covers sounds in the middle range and leaves a low rumble or a high squeak more audible than pink noise would. The pink and brown noise pages go into those colours in depth, and the page about the colors of noise has the full chart.

Is green noise the sound of nature?

Green noise is not the sound of nature in any scientific sense; the phrase is a marketing description. It traces back to the idea of the background noise of the world in the 1996 FAQ, and it spread because the colour green suggests forests and fields.

Real nature sounds are something else: rain, wind, water and birdsong, each with its own pattern of change over time. There is research on those. A 2021 meta-analysis in PNAS (Buxton and colleagues) found natural sounds linked with lower stress and better mood across 18 studies, but it studied recordings of nature, not green noise. The rain, ocean and wind sounds in the generator have their own pages.

If what attracts you to green noise is the idea of nature, you may prefer to mix it with the stream or wind sounds, or to use a nature sound directly.

Is green noise good for sleep?

Some people use green noise to fall asleep because it is softer than white noise and less heavy than brown. No studies have tested green noise for sleep.

What applies is the general evidence on steady noise: it works by masking, it has helped most clearly in noisy settings such as hospital wards and loud city bedrooms, and a systematic review (Riedy and colleagues, 2021) rated the overall evidence for continuous noise as a sleep aid as very low quality. Those studies are described on the white noise page. Green noise shares the masking mechanism but covers a narrower range, so it suits rooms where the disturbing sounds are in the middle range rather than deep traffic rumble.

Which colour is best for sleep has not been tested head to head. If green is the one you find most pleasant at a low volume, that is a sound reason to choose it.

How to use green noise for sleep, focus or relaxation

Use green noise like any steady background sound: low, at a distance, and with a timer when you only need it for a while.

• For falling asleep: start green noise at a low level, speaker away from your head, and set a 30- or 60-minute timer. The WHO recommends that continuous noise in a bedroom stay below about 30 dB.
• For a noisy night: if low traffic rumble gets through, mix in some brown noise; if high sounds get through, add a little pink or white. Green alone leaves both ends open.
• For focus: try it for one working session and compare with a session in quiet. No study has tested green noise for focus; the attention studies used white and pink noise.
• For relaxation or meditation: add the stream or wind sound at a lower level than the noise, so the steady middle is kept and some movement is added.
• In headphones: keep the level where you can still hear someone speaking to you. The WHO safe-listening limit for adults is 80 dB for 40 hours a week.

The mixer on this page has seven noise colours and seven nature sounds, each with its own volume, a set of ready mixes and a timer from 15 minutes to 8 hours that fades the sound out over the last minute.

How to make green noise yourself

You can make green noise from pink noise with any equaliser, and the recipe is short. Start with pink noise, reduce the lows below roughly 100–150 Hz, reduce the highs above roughly 2,000–2,500 Hz, and add a broad boost of about 5 to 8 dB around 500 Hz. Our generator uses 120 Hz, 2,200 Hz and 7 dB.

Two things change the character most. A narrow boost makes the sound hum and draw attention; a broad one keeps it natural. Cutting the highs too far makes it muffled, which some people like for sleep and others find stuffy. Because there is no standard, the right setting is the one that disappears for you after a minute of listening.

If you start from white noise instead of pink, the result will be brighter, because white has much more energy in the upper octaves. Starting from brown gives a darker, heavier version.

Green noise claims to be careful with

Green noise is often sold with claims that go beyond anything measured. The calm way to read them is to separate description from evidence.

• Claim: green noise is a scientific colour of nature. It has no standard definition in acoustics; it is a name for pink-like noise with more energy around 500 Hz.
• Claim: green noise is proven better for sleep than white, pink or brown. No study has tested green noise, and no study has compared the colours head to head for sleep.
• Claim: 500 Hz is a special healing frequency. The 500 Hz figure comes from a 1996 newsgroup FAQ describing a sound, not from research on health.
• Description that holds: green noise is softer than white noise and lighter than brown, and many people find it pleasant.