What pink noise is

The Sleep Foundation defines pink noise as a sound containing a random assortment of all the audible frequencies with more power in the lower frequencies, and gives the specific figure: the volume of individual pitches decreases by 3 decibels with each higher octave. Engineers call the same thing 1/f noise, after the shape of its power spectrum. Stanford's Center for Computer Research in Music and Acoustics describes pink noise as an interesting case because it occurs often in nature, notes the power spectrum rolls off at approximately 3 decibels per octave, and adds a detail worth knowing: the ideal pink filter is an irrational function, so no exact finite-order filter produces it from white noise. Everything sold as pink noise is an approximation of that curve.

What it sounds like

Familiar, mostly. The Sleep Foundation's examples are rain, a waterfall, wind, a river and ocean waves, and it notes that many songs and segments of speech contain elements of pink noise, along with traffic. The Cleveland Clinic reaches for the same image, saying the best comparison is probably that pink noise is the sound of a waterfall, and adds ocean waves, rushing rivers, rustling leaves and rainfall. Set beside white noise the contrast is clear: the Sleep Foundation describes pink noise as perceived as softer, while white noise sounds higher-pitched, like a humming air conditioner. That perceived softness is the whole reason pink noise turns up in sleep research more often than the other colors do.

What researchers have actually tested

The most cited result is narrow and specific. In a 2017 study published in Frontiers in Human Neuroscience, 13 cognitively healthy older adults aged 60 to 84 spent one night with acoustic stimulation and one night with sham stimulation. The stimulation was not a continuous track: it was 50 millisecond pulses of pink noise delivered in blocks of five, timed to the slow oscillations of deep sleep. Slow wave activity increased during stimulation, and overnight improvement on a word-pair recall task averaged 9.2 words after stimulation against 3.1 words after sham. That is a genuine finding in a small sample using lab-timed pulses, and it is not the same as evidence that playing pink noise from a phone all night improves memory.

The honest read

Pink noise has some sleep research behind it, and it is still early. In its sleep-focused article, the Cleveland Clinic notes there have not been research studies comparing pink and white noise directly, which makes the common sleep claim that pink is better than white an assumption rather than a result. The same source is blunt about scope: pink noise is not a magic bullet to cure bad sleep hygiene, and sleep duration, a consistent schedule and caffeine are the things to address alongside any sound. None of that makes pink noise a bad choice. It makes it a preference with some interesting research behind it rather than a treatment.

In short

Pink noise steps down about 3 decibels per octave, which is why it sounds softer than white noise.
The usual comparisons are a waterfall, steady rain, wind and ocean waves.
A 2017 study of 13 adults aged 60 to 84 used timed 50 millisecond pink noise pulses, not a continuous track.
In that study, overnight word recall improved by 9.2 words with stimulation against 3.1 with sham.
Cleveland Clinic notes no sleep research directly comparing pink and white noise.

Related guides

Noise colors explained (white, pink, brown): Noise colors describe how a sound's energy is distributed across frequency.
Pink noise vs white noise: White noise holds every audible frequency at equal power, so it sounds higher-pitched, like a humming air conditioner.
Pink noise for sleep: Pink noise has several small sleep studies behind it, and the findings are promising rather than settled.