The color of noise refers to the power spectrum of a noise signal as a function of frequency. Here, “noise” is either ambient sound or else unwanted sounds in an audio signal. The colors of noise include white, pink, brown, violet, gray, red, green, and black. Noise colors are loosely analogous to the spectrum of light. For example, representing the pattern of white noise as light would be akin to white light, the pattern of blue noise would be blue light, and so on. The exception is brown light.
Here is a look at the colors of sound, examples of familiar sources, and health effects or uses of each color.
White noise is analogous to white light. All the colors of the spectrum occur in white light. Similarly, white noise has equal power density at all frequencies. In other words, it has a flat power spectrum (unlike white light). However, the human ear is not equally sensitive to all frequencies, so white noise sounds bright, with an emphasis on higher frequency tones.
Examples of nearly white noise are television static, radio static, and the sound of a fan.
White noise is a statistical model for noise in acoustical engineering, statistics, physics, and telecommunications. Studies indicate it helps mask tinnitus. Some people find white noise improves cognitive function and masks undesirable background noise, but research shows that not everyone gains a benefit. Some people find white noise distracting and may suffer decreased cognition and neurological health.
Pink Noise or 1/f Noise
Pink noise has equal energy per octave. Its power decreases logarithmically per octave. In other words, power is inversely proportional to frequency. Because of the way the ear works, pink noise sounds like all the frequencies are equally loud.
Pink noise is common in biological systems. Examples of pink noise include the sounds of the human heartbeat, rain, or wind. Electronic devices also emit pink noise from fluctuations in condensed matter materials.
Some sound generators produce pink noise rather than white noise. Its effects on the human body are comparable to those of white noise. However, pink noise is more likely than white noise to aid relaxation and sleep.
Brown Noise or Brownian Noise
The power density of brown noise decreases as the frequency increases, where power density is proportional to 1/f2. The rate of decrease is 6.02 dB per octave. Brown noise does not correspond to a power spectrum representing the color “brown.” Instead, it is a shortened form of Brownian motion or Brownian noise.
A familiar example of brown noise is the sound of distant, rolling thunder. This type of noise aids mental focus and helps regulate sleep patterns in some individuals.
Blue Noise or Azure Noise
Blue or azure noise has a power density that is directly proportional to frequency, f. Its power density increases 3.01 dB per increase in octave.
Blue frequency noise find use in dithering. An example of blue noise is Cherenkov radiation, which is the electromagnetic radiation a charged particle releases as it passes through a dielectric medium. Cherenkov radiation also looks blue. A similar and more familiar sound is a hissing garden hose.
Violet Noise or Purple Noise
Violet or purple noise has power density increasing with frequency, where density is proportional to f2. This is a density increase of 6.02 dB per octave. Another name for violet noise is differentiated white noise.
Violet noise finds use in dithering. This is because the human ear has reduced sensitivity to high-frequency hissing and its easily removed using high-pass filters.
An example of violet noise is the background sound of hydrophones. The sound of a fully open water faucet approximates violet noise.
Gray Noise or Grey Noise
Gray or grey noise is white noise that sounds equally loud at all frequencies. Basically, this boosts the low frequency or bass signal and higher frequencies, compensating for the way the human ear works. Most people find it more pleasing than white noise.
Other Colors of Noise
Some other colors lack precise definitions or else appear to be synonymous to other defined colors:
There are three different descriptions for red noise:
- It is a synonym for brown noise.
- Red noise is like pink noise, but with a different spectrum (1/f for pink noise, but 1/f2 for red noise).
- It describes any power system where power density decreases as frequency increases.
Green noise has multiple definitions, but generally restricts the audio spectrum to a certain frequency range.
- It is the mid-frequency component of white noise.
- It is noise in the vocal spectrum.
- Green noise is a bounded region of Brownian noise.
- It is ambient background noise of the natural world, without anthropogenic sounds.
Definitions for black noise range from the complete absence of sound to noise only at low frequencies.
- It is silence.
- Black noise is infrasonic or below the range of human hearing.
- It is noise with a 1/fβ spectrum, where β > 2.
- It is the lack of noise, except for a few random white noise spikes.
- Black noise is the spectrum of blackbody radiation or thermal noise. In this case, it is noise above the limit of human hearing.
Sometimes even noise is noisy. For example, a noisy white signal that encodes a white image that contains some noise. Similarly, a noisy black signal encodes a noisy black image. These terms find use in telecommunications and don’t usually describe audio signals.
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