White Noise: Why White?

I'm always surprised when I hear the term white noise.

White noise itself sounds a little more "evil" than anything else, I would almost expect it to be called black noise.

• Why is white noise called white noise?

White noise, is called white in analogy to white light:

• The background noise that is continuously present on electrical circuits or radio circuits due to the thermal agitation of electrons.White noise has a flat power spectral density, which is to say that it has equal power at any frequency in any given frequency band.

• The term white noise comes from the fact that it is analogous to white light, which is a combination of all frequencies or wavelengths in the visible light spectrum. (Webster's New World Telecom Dictionary)

From: Atlantic Monthly Company, 1946

• The purest and simplest noise of all, it should be noted, is what the scientist calls a white noise. Like white light, which Newton showed to be compounded of all the colors, white noise is the simultaneous sounding of all the pitches.

Noise is called "white" when it has the same intensity at every frequency. Its name is derived by analogy to light, which is called "white" when it contains all visible frequencies.

Note that there are other "colors" of noise, also named by analogy to frequencies of light:

• Red/brown noise is weighted towards the bass; pink noise is less so.
• Violet noise is weighted towards the treble; blue noise is less so.
• Grey noise is noise that is perceived as white by the human ear.

In informal usage, there is also "black" noise, whose definition is context-sensitive:

Black noise is also called silent noise.

1. Silence
2. Noise with a 1/ƒ^β spectrum, where β > 2. This formula is used to model the frequency of natural disasters.
3. Noise that 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.
4. Noise with a spectrum corresponding to the blackbody radiation (thermal noise). For temperatures higher than about 3×10⁻⁷ K the peak of the blackbody spectrum is above the upper limit to human Hearing range. In those situations, for the purposes of what is heard, black noise is well approximated as violet noise.