Pink noise is a random signal, filtered to have equal energy per octave. In order to keep the energy constant over octaves, the spectral density needs to decrease as the frequency (f) increases. This explains why pink noise is sometimes referred as "1/f noise." In terms of decibels, this decrease corresponds to 3 dB per octave on the magnitude spectrum.
The name of the color comes from visible light that turns pink when a similar spectral distribution is applied.
An octave is a doubling of the frequency and is representative of how our hearing works. For example, the interval between 20 Hz and 40 Hz (the first octave of our hearing range) will be perceived as wide as the interval between 10,000 Hz and 20,000 Hz (the last octave of our hearing range).
Pink noise has equal power in proportionally wide bandwidths. For example, the 20 Hz bandwidth between 20 Hz and 40 Hz (one octave) contains the same amount of sound power as the 10,000 Hz bandwidth between 10,000 Hz and 20,000 Hz (one octave).
Pink noise has a spectral envelope that is not flat within a frequency but rolls off at higher frequencies. Pink noise has a greater relative proportion of low frequency energy than white noise and sounds less “hissy.”
For the human auditory system - which processes frequencies logarithmically - pink noise is supposed to sound even across all frequencies, and therefore best approximates the average spectral distribution of music.
In practice though, it turns out that our ears are more sensitive to certain frequencies, such as in the 2–4 kHz range. Pink noise, despite of its even frequency distribution in the logarithmic frequency scale, will therefore be perceived as colored, with a prominent peak perceived around 3 kHz. Flattening a noise in a perceptual way, will generate grey noise
In audio applications, pink noise is used as a reference tone to check frequency responses and becomes particularly useful when coupled with a 1/3 octave spectrum analyzer.
This type of analyzer operates with a constant percentage bandwidth, which means that the bandwidth of its filters gets wider toward the high frequencies. As the filters get wider, the power of the source signal should decrease, to keep the readings correct, hence, the use of pink noise. If your spectrum analyzer does operate on a constant bandwidth (not a constant percentage), use white noise instead.
Pink noise can be used to measure the adverse effects of room modes as well, although a low frequency sine sweep will be better for such a purpose.
In healthcare applications, pink noise is used to treat hyperacusis, an increased sensitivity to normal environmental sounds, or to mask tinnitus, a ringing in your ear occurring without any stimulus.
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