Noise is an electrical signal that produces an acoustic sound. Colors are assigned to noise and are matched up based, loosely, on the correlations between the sound wave frequency and the color light wave frequency.
Sounds are separated from one another by a statistical characteristic called spectral density. Spectral density is the power distribution in the frequency spectrum. Spectral density is a property that can be used to distinguish different types of noise. The classification by spectral density is given color terminology. Different density spreads are each given different color names to include, most commonly, white, pink, gray, black, brown, blue and violet. These noise colors can be used in a variety of different situations such as acoustics testing, sleep enhancement, physics or electrical engineering as follows:
White Noise
White noise has equal energy per cycle, which means that its frequency spectrum is completely flat. The band of signal between 20 and 40Hz contains the exact same power as the band between 2000 and 2020HZ. It’s referred to as “white” noise because of its analogy to white light where every band of the spectrum shines equally brightly combining to produce white light.
White noise is mainly used for testing audio equipment. It ability to cut through background noise means that its easy to locate the source. It’s also used in audio synthesis to create percussion instruments like cymbals which produce a lot of noise.
Most people are familiar with white noise and its use in environmental settings to aid sleep. White noise overrides background noise. White noise is particularly helpful as a background noise to help babies sleep more restfully and for longer periods of time. Babies are born accustomed to the very loud sounds of the womb. Normal noises, or the absence of loud sounds, after the more familiar loud sounds of the womb can be very stressful to babies. White noise gives them a sense of the safety of the womb and they sleep more restfully. White noise can help you sleep better as well by masking sounds such as outside traffic sounds, passing airplanes, a television droning, etc., that might disturb your sleep. There’s no need to invest in expensive white noise machines to get the desired white noise result. The static that can be heard on the frequencies between radio stations is white noise. Turn on the radio at a station set to static and increase the volume to get an adequate level of white noise.
White Noise
While white noise has equal power in frequency bands of equal width, pink noise’s spectrum falls off logarithmically over time, with equal power in bands that are proportionally wide. The band of signal between 20 and 40Hz has the same power in it as the band between 2000 and 4000Hz.
Pink noise is useful because it’s the same as humans hear. Each octave in the western musical scale contains the same amount of energy as the octave above and below it, despite encompassing a wider frequency range. Pink noise replicates that circumstance so that it is very useful as a signal to test amplifiers and loudspeakers.
Brownian/Red Noise
The brown noise is a low frequency infra sound that disrupts the movement of the colon because it sends signals in the same frequency that the colon and bowels use. Brown and red noise are the same thing and more commonly referred to, after Robert Brown, as Brownian noise. It is heavily weighted on the lower end of the density spectrum and has a sound like a low roar. It does not include a constant component or value at zero density. Loosely used, “red” noise could apply to any system where power density decreases when frequency increases.
It’s also known as “random walk” or “drunkard’s walk” noise. Its the type of signal produced by Brownian motion. Brownian motion is the random movement of microscopic particles suspended in a liquid or gas, caused by collisions with molecules of the surrounding medium.
Brownian noise is used in climatology, with arguable value, to describe shifts in climate regime.
Grey Noise
Grey noise is random pink noise subjected to a psycho acoustic equal loudness curve (such as an inverted A-weighting curve) over a given range of frequencies. This causes the listener to perceive that the noise is equally loud at all frequencies.
“Grey noise then, has a sound frequency that, if translated to a light frequency, would roughly be the color grey. Grey noise is a random sound that is distinguishable from other types of sound because it sounds the same, to the human ear, at all frequencies.
Perhaps the most commonly discussed color of noise is white noise. Also a random noise, white noise is distinguishable not only because of its color but because it actually is the same at all frequencies. That is, it has the same energy levels at all frequency levels. Grey noise does not, but sounds that way to the human ear.
The reason grey noise sounds the same to the human ear regardless of the frequency is because of something called the psychoacoustic equal loudness curve. Also called the psychoacoustic equal loudness contour or Fletcher-Munson curve, this quality within grey noise, takes the individual biases of the human ear into account, measuring the loudness, level, and intensity of sound. The result is a sound that seems to sound the same at all frequencies.”
Blue and Violet Noise
Blue and violet noise is on the other end of the spectrum from red and brown noise. It’s similar to pink noise in that it changes proportionally with frequency increases, but it increases, rather than decreases. A number of studies have shown that the arrangement of retinal cells in a blue-noise pattern results in good visual resolution.
Violet noise is likened to an inverted version of brownian noise. Its power density increases per octave with increasing frequency over a finite frequency range. It can be used as a treatment for tinnitus because its high frequencies can mask the high frequencies heard by many who suffer with tinnitus.
Others
There is a small number of less tightly-defined colors of noise with no widely accepted meaning. Orange noise is a spectrum that has a few bands centered around the frequencies of exact musical notes eliminated entirely. This sound color plays everything that isn’t perfectly in tune, and nothing that is. These “sour” notes give the noise its name. It can be used to check tuning.
Black noise, which has a number of different explanations. It is sometimes defined as pure silence, but other describe it as outputting active noise control that cancels out existing noise essentially resulting in negative noise. A maker of an ultrasonic vermin repeller calls it white noise that is silent below 20,000Hz.
Last but not least, green noise is described as “either the mid-point of the white noise spectrum, or the background noise of the world – as a power spectrum that’s averaged over a very long time in several outdoor sites around the planet. It looks rather like pink noise, but with a hump added around 500Hz.”
In addition to these more technical applications of sound colors, there is considered to be a direct relationship between colors, sound and our health. The ColorOfSound.com web site provides a very interesting exploratory look at how the interrelationship between colors and sounds relates to our health.
