From AC power hum to synesthesia

100/100: colored noise:
Name: Mark Thompson #9
Date: 1:00 pm Wed Dec 27, 1989

I work with colored noise occasionally, and recently made several tapes utilizing psychoacoustic noise effects.

Here's a rundown of some interesting varieties of "noise":

White Noise: This kind of noise has equal energy (loudness, or volume) over frequency -- that is, if you measure the amplitude of the sound from 100 to 200 Hz (Hz means "Hertz" or "cycles per second") that chunk of the frequency spectrum will have the same sound amplitude as a chunk from 3000 to 3100 Hz or even 20,000 to 20,100 Hz. White noise is kind of "bright" and not terribly relaxing, but is very effective for masking other sounds, and has been shown to promote auditory hallucinations under certain circumstances.

Pink Noise: This noise has equal energy PER OCTAVE. This means that the volume decreases logarithmically with frequency. Usually pink noise is made by low-pass filtering white noise. For comparison, pink noise will have the same sound amplitude
from 100-200Hz that it does from 200-400Hz or 10,000-20,000Hz. Pink noise sounds more natural than white noise (it sounds like rushing water or ocean surf) and is quite relaxing. It's often used for ambience in electronic music, and as a test signal for "tuning" sound reenforcement systems (many equalizers and audio spectrum analyzers have built-in pink noise generators).

Red Noise: A very bassy (heavily low-pass filtered) kind of noise. This sounds like a low rumble - a subway train going by or a noisy air-conditioning system. The definition of red noise is not as precise as that of white and pink noise, and the term mostly refers to low-pitched noises used for electonic music.

Brown Noise: This is a special kind of noise that has a 1/f amplitude distribution - the volume is inversely proportional to pitch. It has the special property of being a "fractal" or statistically self-similar waveform. No matter how far you zoom in on the wave with an oscilloscope, the waveform has the same "texture". Brown noise is also the sound made by a "random walk" which makes the amplitude of a waveform travel up and down at random. The pitch motion of most musical melodies have a 1/f distribution, which is more a measure of melodic texture than a commentary on the musical experience.

Blue Noise: This is just high-pass filtered white noise. It sounds really screechy and artificial. Like "Red Noise" this is not a precise term, and just refers to noises with a lot of treble tones.

Diotic Noise: This refers to the stereophonic or binaural properties of a noise signal. Diotic noise is the same noise signal presented to both ears - monaural noise. Diotic noise can be either in-phase or phase-reversed. In-phase noise sounds like it is coming from inside one's head when heard through headphones. 180-degree phase-reversed noise sounds like it is coming from "all around" one's head when heard through headphones.

Dichotic Noise: This is two separate, unrelated noise signals presented to each ear. It's much more stimulating and refreshing (subjectively) than monaural or diotic noise. Where monaural noise sounds like radio hiss, dichotic noise sounds like being outdoors in a rainstorm.

Taken from: http://mv.lycaeum.org/M2/noise_ahf.html

Others

There are also many "less official" colors, usually with multiple definitions

Red noise

1. A synonym for Brownian noise, as above
2. A synonym for pink noise, as above

Orange noise

"Orange noise is quasi-stationary noise with a finite power spectrum with a finite number of small bands of zero energy dispersed throughout a continuous spectrum. These bands of zero energy are centered about the frequencies of musical notes in whatever scale is of interest. Since all in-tune musical notes are eliminated, the remaining spectrum could be said to consist of sour, citrus, or "orange" notes."

Green noise

1. "Green noise is supposedly the background noise of the world. A really long term power spectrum averaged over several outdoor sites. Rather like pink noise with a hump added around 500 Hz."
2. The mid-frequency component of white noise, used in halftone dithering[12]
3. Bounded Brownian noise

Black noise

Black noise, or silent noise, has several different definitions:

1. Silence
2. Noise with a 1/fβ spectrum, where β > 2 (Manfred Schroeder, "Fractals, chaos, power laws"). Used in modeling various environmental processes. Is said to be a haracteristic of "natural and unnatural catastrophes like floods, droughts, bear markets, and various outrageous outages, such as those of electrical power." Further, "because of their black spectra, such disasters often come in clusters."
3. Noise that has a frequency spectrum of predominantly zero power level over all frequencies except for a few narrow bands or spikes. Note: An example of black noise in a facsimile transmission system is the spectrum that might be obtained when scanning a black area in which there are a few random white spots. Thus, in the time domain, a few random pulses occur while scanning.
4. "The output of an active noise control system which cancels an existing noise, leaving the local environment noise free. The comic book character Iron Man used to have a "black light beam" that could darken a room like this, and popular science fiction has a tendency to portray active noise control in this light."[11] The Batman Beyond supervillian Shriek also had a weapon like this, which effectively blocked out all noise.
5. "As seen in the sales literature for an ultrasonic vermin repeller, black noise with a power density that is constant for a finite frequency range above 20 kHz. More accurately, ultrasonic white noise. This black noise is like the so-called black light with frequencies too high to be sensed, but still capable of affecting the environment."

Noisy white

In telecommunication, the term noisy white has the following meanings:

1. In facsimile or display systems, such as television, a nonuniformity in the white area of the image, i.e., document or picture, caused by the presence of noise in the received signal.
2. A signal or signal level that is supposed to represent a white area on the object, but has a noise content sufficient to cause the creation of noticeable black spots on the display surface or record medium.

Noisy black

In telecommunication, the term noisy black has the following meanings:

1. In facsimile or display systems, such as television, a nonuniformity in the black area of the image, i.e., document or picture, caused by the presence of noise in the received signal.
2. A signal or signal level that is supposed to represent a black area on the object, but has a noise content sufficient to cause the creation of noticeable white spots on the display surface or record medium.