Pink noise is a color of noise, not entirely unlike white noise.
Both white noise and pink noise contain all the frequencies that are audible to humans — 20 hertz to 20,000 hertz — but the way their signal power is distributed among those frequencies differs. White noise has equal power per hertz throughout all frequencies, while the power per hertz in pink noise decreases as the frequency increases.
As a result, the lower frequencies in pink noise are louder and have more power than the higher frequencies. However, most people perceive the sound of pink noise as being even, or flat, because it has equal power per octave.
(In acoustics, an octave is a frequency band whose highest frequency is twice its lowest frequency. For example, the band from 20 hertz to 40 hertz is an octave, as is the band from 40 to 80 hertz.)
So though the power per hertz decreases with increasing frequency, the width of successive octaves increases (they contain more frequencies), giving pink noise equal power per octave.
The pattern of pink noise occurs in a number of natural systems, including your daily heartbeat rhythms, quasar luminosity and traffic flow, according to the Gilden Lab, a research center at the University of Austin at Texas.
In terms of applications, pink noise is often used to test and equalize loudspeakers in rooms and auditoriums. In recent years, pink noise has also become popular in business settings — the noise can mask low-frequency background sound, potentially helping to increase productivity and concentration among employees.
In addition to white noise and pink noise, there are other colors of noise, including blue noise and brown noise.