Colours of Noise
The classification of noise by spectral density (power distribution in the frequency spectrum) is given “colour” terminology, with different types named after different colours, and is common in disciplines like acoustics, electrical engineering, and physics. The colour names are derived from a loose analogy between the spectrum of sound wave frequencies (as shown in the blue diagrams) and the equivalent spectrum of light wave frequencies. That means when the sound wave pattern of “blue noise” were translated into light waves, the resulting light would be blue, and so on. The Federal Standard 1037C Telecommunications Glossary defines white, pink, blue, and black.
White noise is a signal, named by analogy to white light, with equal energy per cycle (hertz). Just as white light is a combination of all the different coloured (frequencies) lights, white noise is a combination of all of the different frequencies of sound. This produces a flat frequency spectrum in linear space.
The frequency spectrum of pink noise is flat in logarithmic space; it has equal power in bands that are proportionally wide. The power density, compared with white noise, decreases by 3 dB per octave.
Many people describe white or pink noise as similar to running water, rain, or noise on a vacant TV channel. Most of the “sleep” machines on the market are based on generating variations of white or pink noise.
Blue noise is also called azure noise. Blue noise’s power density increases 3 dB per octave with increasing frequency over a finite frequency range. In computer graphics, the term “blue noise” is sometimes used more loosely as any noise with minimal low frequency components and no concentrated spikes in energy. This can be good noise for dithering; retinal cells are arranged in a blue-noise-like pattern for this reason.
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 has various associations. It has a frequency spectrum of predominantly zero power level over all frequencies except for a few narrow bands or spikes hence also referred to as silence. “The output of an active noise control system which cancels an existing noise, leaving the local environment noise free. 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.” The Batman Beyond supervillian Shriek also had a weapon like this, which effectively blocked out all noise.