The Colors of Noise
We thought this Usenet message quite interesting.
Also, at the end of this page are:
- a comment by J. Russell Lemon regarding green noise
- a comment by Michael Golanbari regarding pink noise
- a comment by David Thall regarding grey (gray) noise
Newsgroups: comp.dsp,comp.sys.ibm.pc.soundcard.tech,comp.speech,alt.sci.physics.acoustics
Subject: The colors of noise 1.3
Date: Mon, 07 Oct 1996 08:22:31 -0400
Organization: Ford Motor Company
Lines: 107
Colors of noise pseudo FAQ, version 1.3
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That email just keeps coming in. So, here's the latest rev. Thanks to the many people who pointed out the flaws in my pink and blue definitions. Thanks Kev fot the pointer to FS-1037C. Due to popular demand, I am reversing my previous stand and adding the definition of orange noise.
The noises are now in spectral order (artistic license has been taken over where white, black, grey, and brown fit into a spectrum). Anyone is welcome to help fill in the gaps. We're up to three defintions of black noise. Keep them coming!
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White noise (common definition) power density is constant over a finite frequency range. AKA Johnson noise.
Pink noise (common definition) power density decreases 3dB per octave with increasing frequency (density proportional to 1/f) over a finite frequency range which does not include DC. Each octave contains the same amount of power. Many point out that this is not a trivial filtering problem. AKA flicker noise.
Red noise (common definition within the oceanographic field, contributed by P.J. "Josh" Rovero) (Anyone have the spectrum?)
oceanic ambient noise (ie, noise distant from the sources) is often described as "red" due to the selective absorption of higher frequencies."
Orange noise (anonymous contribution) (Anyone foolish enough to want the spectrum?)
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 system of music 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 roomfull of primary school students equipped with plastic soprano recorders.
Green noise (defined by some folks producing relaxation tapes, Mystic Moods, I believe) 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 500Hz. (Anyone have the spectrum?)
Blue noise (FS-1037C) power density increases 3dB per octave with increasing frequency (density proportional to f) over a finite frequency range. This can be good noise for dithering.
Purple noise (origional definition, contributed by Jon Risch) power density increases 6dB per octave with increasing frequency (density proportional to f^2) over a finite frequency range. Differentiated white noise. AKA violet noise.
Grey noise (heard this one a couple of times, but can't put my finger on a source) noise subjected to a psychoacoustic equal loudness curve (such as an inverted a-weight curve) over a given range of frequencies, so that it sounds like it is equally loud at all frequencies. This would be a better definiton of "white noise" than the "equal power at all frequencies" definition, since real "white light" has the power spectrum of a 5400K black body, not an equal power spectrum.
Brown noise (Jon M. Risch, rbmccammon) power density decreases 6dB per octave with increasing frequency (density proportional to 1/f^2) over a frequency range which does not include DC. Is not named for a power spectrum that suggests the color brown, rather, the name is a coruption of Brownian motion. If we were going to pick a color, red might be good since pink noise lies between this noise and white noise. Unfortuantly, red is already taken. AKA "random walk" or "drunkard's walk" noise.
**** Three different definitions of black (silent) noise ****
Black noise (contributed by Jeff Mercure, his own definition) whatever comes out of an active noise control system and cancles an existing noise, leaving the world 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 an annoying tendancy to portray active noise control in this light.)
Black noise (seen in the sales literature for an ultrasonic vermin repeller) power density is constant for a finite frequency range above 20kHz. Ultrasonic white noise. This black noise is like the so-called "black light" with frequencies too high to be preceived as sound, but still capable of affecting you or your surroundings.
Black noise (Manfred Schroeder, "fractals, chaos, power laws," contributed by Mike Arnao)
has an f ^ -beta spectrum, with beta > 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."
--
Joseph S. Wisniewski | Views expressed are my own, and don't reflect
__Ford Motor Company | those of the Ford Motor Co. or affiliates.
____Project Sapphire | Trans Am, Daytona, Bonneville, and IROC are
| just races, won by people driving Ford cars!
Comments on the Colors of Noise
I started using Fortran 4 in 1968 when I was assigned to the Dignital Communications Experimental Facility at Rome Air Development Center at Griffiss AFB, N.Y. It is there I first saw the U. S. Government spec for "Green Noise" which was used to simulate voice bandwidth for testing communication channels. As I remember, it was flat from 500 Hz to about 2 kHz and then rolled off at 9 db/octave. Below 500 Hz it rolled off at 6 db ?? per octave. Not sure. It was a long time ago, far from my home in California.