Blue noise rises about 3 dB per octave and violet noise rises about 6 dB per octave, so both put their power in the high end and sound bright and hissy. Green noise is different: it is an informal name with no standard slope. If you already know the noise colours white, pink and brown, these three sit on the other side of the dial or off to the side of it.
What are blue and violet noise?
Blue noise is noise whose power climbs as the pitch goes up. Its power density increases 3.01 dB per octave, in proportion to frequency, over a finite range of frequencies. Violet noise, also called purple noise, climbs twice as fast: 6.02 dB per octave, in proportion to frequency squared. It is what you get by differentiating white noise.
In practice, more and more of the energy sits in the top octaves. Both sound thin, bright and hissy. Violet is the thinner of the two.
| Colour | Slope | How it sounds |
|---|---|---|
| Brown | −6 dB per octave | Deep and rumbling |
| Pink | −3 dB per octave | Soft and even |
| White | Flat | Bright hiss |
| Blue | +3 dB per octave | Brighter, thinner |
| Violet | +6 dB per octave | Thinnest, hissiest |
What do the numbers mean?
A noise colour is a slope, not a sound you hear directly. The names come from light: white noise has equal energy at every frequency, like white light, and pink noise was named for the pink look of light with its spectrum.
The slope tells you how power changes each time the pitch doubles. Pink noise decreases about 3 dB per octave, so every octave carries the same total energy. Blue noise does the reverse, with +3 dB per octave, and violet goes further. Brown noise falls 6 dB per octave and violet rises 6 dB per octave, so they mirror each other.
White noise is flat per hertz, yet it already sounds bright. The ear hears pitch on a roughly logarithmic scale, and a high octave spans far more hertz than a low one, so white noise packs more energy into the upper octaves. Blue and violet exaggerate that. A small slope compounds quickly. Over five octaves, which spans roughly the range from a low hum to a high hiss, a +6 dB per octave slope adds up to a 30 dB difference between the bottom and the top, which is a large gap in power. That is why violet sounds so much thinner than blue, even though the two numbers look close. For the three colours below white, see white, pink and brown noise.
Is there a green noise?
There's a name, but not a standard. Green noise is an informal term and it means different things in different places. Wikipedia lists several loose meanings: the mid-frequency part of white noise, used in halftone dithering; bounded Brownian noise; and a vocal-spectrum noise used to test audio circuits.
The meaning people meet most often in sound is a description attributed to Joseph S. Wisniewski. He calls it similar to pink noise, but with more energy around 500 Hz, and says ambient sound producers market it as "the background noise of the world". That is a description, not a definition, and nobody publishes a dB per octave figure for it. So if a green noise track claims an exact slope, ask where the number comes from.
Where do people use blue and violet noise?
Mostly in engineering. According to the same Wikipedia article, early digital audio dither often used violet noise, because the ear is less sensitive to high-frequency hiss. In computer graphics, "blue noise" is a looser term for noise with little low-frequency content and no concentrated spikes, and it is used for dithering images.
Neither colour has a body of published sleep research the way pink noise does, so there is no study to point to. People who listen to them for ordinary background sound choose them by taste. If you want a colour shaped around how the ear hears, see grey noise.
How do you try them safely?
Start quiet. Bright noise is mostly high frequencies, and it can sound louder and harsher than a deeper colour at the same setting. The NIDCD says sounds at or below 70 dBA are unlikely to cause hearing loss, while long or repeated exposure at or above 85 dBA can. Normal conversation is about 60 to 70 dBA. The decibel scale is logarithmic, so a rise of 10 dB sounds about twice as loud while carrying ten times the intensity. A small turn of the volume matters more than it feels. Muffled sound, trouble understanding people, or ringing in your ears are signs that the level is too high, and ringing that does not go away is worth a visit to a doctor or audiologist.
To hear the difference, play white noise at a low volume, then switch to blue noise and violet noise without touching the volume. You should hear the low end thin out step by step. Then try brown noise or pink noise at the same volume and notice how much of the hiss disappears. You're not picking a winner. You're hearing what a few dB per octave does to a sound, so the numbers on this page stop being abstract. Hush Machine is coming to iPhone. It makes its noise live on the phone with no loops, so nothing repeats, and a timer and fade let you set how long it plays.
Bottom line
Blue noise rises 3 dB per octave and violet rises 6 dB per octave, so both are bright. Green noise has no standard slope, so be careful with any exact number attached to it.
Questions
- What is the difference between blue and violet noise?
- Both get louder as pitch rises, but violet climbs twice as fast. Blue rises about 3 dB per octave and violet about 6 dB per octave, so violet sounds thinner and hissier.
- Is green noise a real noise colour?
- It is an informal name with no standard slope. One description calls it pink-like noise with more energy around 500 Hz. Other sources use the name for other things, so check what a given source means.
- Is violet noise the opposite of brown noise?
- Roughly, yes. Brown noise falls about 6 dB per octave and violet rises about 6 dB per octave, so one is deep and the other is bright.
- How loud should bright noise be?
- Keep it quiet. NIDCD says sound at or below 70 dBA is unlikely to cause hearing loss, and long or repeated exposure at or above 85 dBA can.