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Pink noise ( sample ( help· info)), also known as 1/f noise, is a signal or process with a frequency spectrum such that the power spectral density is proportional to the reciprocal of the frequency. Sometimes pronounced as one over f noise, it is also called flicker noise. In other words, it is a sound that falls off steadily into the higher frequencies, instead of producing all frequencies equally.
There is equal energy in all octaves. In terms of power at a constant bandwidth, 1/f noise falls off at 3 dB per octave.
The human auditory system, which uses a roughly logarithmic concept of frequency approximated by the Bark scale, does not perceive all audible frequencies with equal sensitivity, signals at about 1 kHz appear to be loudest, the 'loudness' of other frequencies will appear to drop as the frequency changes from from the 1 kHz 'peak'. However, humans may still differentiate between white noise and pink noise with ease.
Graphic equalizers also divide signals into bands logarithmically and report power by octaves; audio engineers put pink noise through a system to test whether it has a flat frequency response in the useful spectrum.
From a practical point of view, producing true pink noise is impossible, since the energy of such a signal would be infinite. That is, the energy of pink noise in any frequency interval from f1 to f2 is proportional to log(f2 / f1) and if f2 is infinity, so is the energy. Practically, pink noise is only pink over a certain frequency interval. The same is true of white noise which is usually used to produce pink noise by filtering to remove more and more energy at succesively higher frequencies (about 3 dB per octave).