Colors of Noise

noise color

Noises are classified based on their spectral properties; they are named for the color they most resemble in the visible light spectrum. If the sound wave pattern of ‘red noise’ were translated into light waves, the resulting light would be red, and so on. White noise is an audio signal that contains all the frequencies audible to the human ear. It is analogous to white light, which contains all the colors of light visible to the human eye.

Pink noise is a signal that is louder at low frequencies and decreases at a constant rate. It is sometimes referred to as flicker noise particularly when it describes background noise emitted by an electronic device. Pink noise is used to make music, sound effects, or merely as a pleasant background sound and is reported to sound more like the ocean than white noise (which is often compared to the sound of rainfall or TV static) because of its bias towards lower frequencies.

 ‘Red noise,’ also called Brown noise or Brownian noise, refers to a system where power density decreases with increasing frequency. ‘Brown’ noise is not named for a power spectrum that suggests the color brown; rather, the name is a corruption of Brownian (random) motion. It is also known as ‘random walk’ or ‘drunkard’s walk’ noise. Unlike pink, red, and brown noise, blue (or azure) noise’s power density increases with increasing frequency. 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 (the intentional use of noise to reduce the error of compression). Retinal cells are arranged in a blue-noise-like pattern which yields good visual resolution.

 Violet (or purple) noise’s power density also increases with increasing frequency. It is also known as differentiated white noise, due to its being the result of the differentiation of a white noise signal. Grey noise is random white noise subjected to a psychoacoustic equal loudness curve over a given range of frequencies, giving the listener the perception that it is equally loud at all frequencies. This is in contrast to standard white noise which has equal strength over a linear scale of frequencies but is not perceived as being equally loud due to biases in the human equal-loudness contour.

There are also many ‘unofficial’ colors, usually with multiple definitions. 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 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 is also called silent noise. It 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. ‘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 at one time possessed a ‘black light beam’ that could darken a room in this manner, and popular science fiction has a tendency to portray active noise control in this light.’ Black noise is more accurately described as 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.’ Black noise also refers to noise with a spectrum corresponding to the blackbody radiation (thermal noise).

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