Pink Noise

Pink noise or 1/ƒ noise (sometimes also called flicker noise) is a signal or process with a frequency spectrum such that the power spectral density (energy or power per Hz) is inversely proportional to the frequency. In pink noise, each octave carries an equal amount of noise power. The name arises from the pink appearance of visible light with this power spectrum.

Within the scientific literature the term 1/ƒ noise is sometimes used a little more loosely to refer to any noise with a power spectral density of the form

where ƒ is frequency and 0 < α < 2, with α usually close to 1. These "1/ƒ-like" noises occur widely in nature and are a source of considerable interest in many fields. The distinction between the noises with α near 1 and those with a broad range of α approximately corresponds to a much more basic distinction. The former (narrow sense) generally come from condensed matter systems in quasi-equilibrium, as discussed below. The latter (broader sense) generally correspond to wide range of non-equilibrium driven dynamical systems.

The term flicker noise is sometimes used to refer to 1/ƒ noise, although this is more properly applied only to its occurrence in electronic devices due to a direct current. Mandelbrot and Van Ness proposed the name fractional noise (sometimes since called fractal noise) to emphasize that the exponent of the spectrum could take non-integer values and be closely related to fractional Brownian motion, but the term is very rarely used.