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White noise

White noise is a signal (or process), named by analogy to white light, with equal energy per cycle (hertz).This produces a flat frequency spectrum in linear space. In other words, the signal has equal power in any band of a given bandwidth (power spectral density). For example, the range of frequencies between 40 Hz and 60 Hz contains the same amount of sound power as the range between 4000 Hz and 4020 Hz has.
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 considered "white" if it has a flat spectrum over a defined frequency band (such as the range of human hearing, or the frequency response of audio/visual equipment).


Pink noise

The frequency spectrum of pink noise is flat in logarithmic space; it has equal power in bands that are proportionally wide. This means that pink noise would have equal power in the frequency range from 40 to 60 Hz as in the band from 4000 to 6000 Hz. Since humans hear in such a proportional space, where a doubling of frequency is perceived the same regardless of actual frequency (40–60 Hz is heard as the same interval and distance as 4000–6000 Hz), every octave contains the same amount of energy and thus pink noise is often used as a reference signal in audio engineering. That is, the human auditory system perceives approximately equal magnitude on all frequencies. The power density, compared with white noise, decreases by 3 dB per octave (density proportional to 1/f ). For this reason, pink noise is often called "1/f noise".
Since there are an infinite number of logarithmic bands at both the low frequency (DC) and high frequency ends of the spectrum, any finite energy spectrum must have less energy than pink noise at both ends. Pink noise is the only power-law spectral density that has this property: all steeper power-law spectra are infinite if integrated to the DC, low frequency end, and all flatter power-law spectra are infinite if integrated to the high-frequency limit.


Brown noise

In fields that adopt precise definitions, the terminology "red noise", also called Brown noise or Brownian noise, will usually refer to a power density which decreases 6 dB per octave with increasing frequency (density proportional to 1/f 2) over a frequency range which does not include DC (in a general sense, does not include a constant component, or value at zero frequency). In areas where terminology is used loosely, "red noise" may refer to any system where power density decreases with increasing frequency.
The first definition can be generated by an algorithm which simulates Brownian motion or by integrating white noise. "Brown" noise is not named for a power spectrum that suggests the color brown; rather, the name is a corruption of Brownian motion. "Red noise" describes the shape of the power spectrum, with pink being between red and white. Also known as "random walk" or "drunkard's walk" noise.


Blue noise

Blue noise is also called azure noise. Blue noise's power density increases 3 dB per octave with increasing frequency (density proportional to f ) 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.


Violet noise

Violet noise is also called purple noise. Violet noise's power density increases 6 dB per octave with increasing frequency (density proportional to f 2) over a finite frequency range. It is also known as differentiated white noise.


Grey noise

Grey noise is random pink noise subjected to a psychoacoustic equal loudness curve (such as an inverted A-weighting 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 pink noise which has equal strength over a logarithmic scale of frequencies but is not perceived as being equally loud due to biases in the human equal-loudness contour.


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
3. Bounded Brownian noise


Black noise
Black noise is silence.