There are many forms of noise with various frequency characteristics that are classified by "color". Some have well-defined technical definitions, while others are colloquial or jokes.

Technically defined

The most commonly used is white noise, a signal (or process) with a flat frequency spectrum. In other words, the signal has equal power in any band, at any center frequency, having a given bandwidth.

An infinite-bandwidth white noise signal is purely a theoretical construct. By having power at all frequencies, the total power of such a signal would be infinite. In practice, a signal is "white" if it has a flat spectrum over a defined frequency band.

The next most commonly used color is pink noise. Its frequency spectrum is not flat, but has equal power in bands that are proportionally wide. (As an example, white noise would have an equal amount of power in the band from 1 to 2 kHz as in the band from 2 to 3 kHz. Pink noise would have equal power in the band from 1 to 2 kHz as in the band from 2 to 4 kHz.) Pink noise is perceptually white. That is, the human auditory system perceives approximately equal magnitude on all frequencies. The power density decreases by -3 dB per octave (density proportional to 1/f).

Brown noise is similar to pink noise, but with a power density decrease of -6 dB per octave with increasing frequency (density proportional to 1/f²) over a frequency range which does not include DC. It can be generated by an algorithm which simulates Brownian motion. Brown noise is not named for a power spectrum that suggests the color brown; rather, the name is a corruption of Brownian motion. Also known as "random walk" or "drunkard's walk" noise. (Not to be confused with the infrasonic brown note which is rumored to cause people to lose control of their bowels.)

Blue noise 's (FS-1037C) power density increases 3 dB per octave with increasing frequency (density proportional to f) over a finite frequency range. This can be good noise for dithering.

Purple noise 's power density increases 6 dB per octave with increasing frequency (density proportional to f²) over a finite frequency range. Differentiated white noise. Also known as violet noise.

Grey noise is noise subjected to a psychoacoustic equal loudness curve (such as an inverted A-weighting curve) over a given range of frequencies, so that it sounds like it is equally loud at all frequencies. Some say this would be a better definition of "white noise" than the "equal power at all frequencies" definition, since real "white light" has the power spectrum of a 5400 K black body, not an equal power spectrum.

"Less official"

There are also many "less official" colors.

Red noise (common definition within the oceanographic field, contributed by P.J. "Josh" Rovero) is oceanic ambient noise (i.e., noise distant from the sources) is often described as "red" due to the selective absorption of higher frequencies by the ocean.

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. "Orange noise is most easily generated by a roomful of primary school students equipped with plastic soprano recorders."

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.

Black noise, or silent noise, has at least three different definitions:

1. Whatever comes out of an active noise control system and cancels an existing noise, leaving the world noise free. The comic book character Iron Man used to have a "black light beam" that could darken a room like this, and popular sci-fi has a tendency to portray active noise control in this light.
2. 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.
3. (Manfred Schroeder, "Fractals, chaos, power laws") has an f^−β spectrum, with β > 2, and is characteristic 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."

External links

• The colors of noise