Brown noise, also called Brownian or red noise, has power that falls by about 6 dB per octave, twice the slope of pink noise, so its energy is concentrated in the low frequencies. The result is a deep, soft rumble, much darker than the hiss of white noise. The name comes not from a colour but from Brownian motion, the random jiggling of particles described by the botanist Robert Brown, because the signal follows the same kind of random walk.
Public interest in brown noise has grown quickly in recent years, with many people saying it helps them focus or fall asleep. Scientific support is thin: a 2024 meta-analysis on noise and attention in ADHD could not find a single study of brown noise, and a systematic review of continuous noise as a sleep aid rated the evidence for such noise in general as very low quality.
It can be a pleasant masking sound at a low volume, but it is not a treatment; persistent sleep or focus problems are worth raising with a doctor.