Pink noise
Noise with equal energy in every octave — what sounds even to an ear that thinks in octaves rather than hertz.
White noise has equal power per hertz. Since the octave from 10 to 20 kHz holds a thousand times more hertz than 10 to 20 Hz, it sounds harsh and fizzy. Pink noise falls 3 dB per octave, its power density proportional to 1/f, and comes across as an even hiss or a distant waterfall. Brown noise falls twice as fast and sounds like thunder behind a wall; blue noise tilts the other way.
Audio engineers use it to calibrate speakers and measure rooms: play pink noise, read a third-octave real-time analyser, and a flat line means even reproduction. The 1/f pattern also turns up in nature, from resistor noise to heart rhythms and melodies, which has produced more speculative papers than practical conclusions.