100/100: colored noise: Name: Mark Thompson #9 Date: 1:00 pm Wed Dec 27, 1989 I work with colored noise occasionally, and recently made several tapes utilizing psychoacoustic noise effects. Here's a rundown of some interesting varieties of "noise": White Noise: This kind of noise has equal energy (loudness, or volume) over frequency -- that is, if you measure the amplitude of the sound from 100 to 200 Hz (Hz means "Hertz" or "cycles per second") that chunk of the frequency spectrum will have the same sound amplitude as a chunk from 3000 to 3100 Hz or even 20,000 to 20,100 Hz. White noise is kind of "bright" and not terribly relaxing, but is very effective for masking other sounds, and has been shown to promote auditory hallucinations under certain circumstances. Pink Noise: This noise has equal energy PER OCTAVE. This means that the volume decreases logarithmically with frequency. Usually pink noise is made by low-pass filtering white noise. For comparison, pink noise will have the same sound amplitude from 100-200Hz that it does from 200-400Hz or 10,000-20,000Hz. Pink noise sounds more natural than white noise (it sounds like rushing water or ocean surf) and is quite relaxing. It's often used for ambience in electronic music, and as a test signal for "tuning" sound reenforcement systems (many equalizers and audio spectrum analyzers have built-in pink noise generators). Red Noise: A very bassy (heavily low-pass filtered) kind of noise. This sounds like a low rumble - a subway train going by or a noisy air-conditioning system. The definition of red noise is not as precise as that of white and pink noise, and the term mostly refers to low-pitched noises used for electonic music. Brown Noise: This is a special kind of noise that has a 1/f amplitude distribution - the volume is inversely proportional to pitch. It has the special property of being a "fractal" or statistically self-similar waveform. No matter how far you zoom in on the wave with an oscilloscope, the waveform has the same "texture". Brown noise is also the sound made by a "random walk" which makes the amplitude of a waveform travel up and down at random. The pitch motion of most musical melodies have a 1/f distribution, which is more a measure of melodic texture than a commentary on the musical experience. Blue Noise: This is just high-pass filtered white noise. It sounds really screechy and artificial. Like "Red Noise" this is not a precise term, and just refers to noises with a lot of treble tones. Diotic Noise: This refers to the stereophonic or binaural properties of a noise signal. Diotic noise is the same noise signal presented to both ears - monaural noise. Diotic noise can be either in-phase or phase-reversed. In-phase noise sounds like it is coming from inside one's head when heard through headphones. 180-degree phase-reversed noise sounds like it is coming from "all around" one's head when heard through headphones. Dichotic Noise: This is two separate, unrelated noise signals presented to each ear. It's much more stimulating and refreshing (subjectively) than monaural or diotic noise. Where monaural noise sounds like radio hiss, dichotic noise sounds like being outdoors in a rainstorm.