Most people are familiar with white noise, that static sound of an air conditioner that lulls us to sleep by drowning out any background noise.
Except technically, the whirl of a fan or hum of the AC isn’t white noise at all. Many of the sounds we associate with white noise are actually pink noise, or brown, or green, or blue. In audio engineering, there’s a whole rainbow of noise colors, each with its own unique properties, that are used to produce music, help relaxation, and describe natural rhythms like the human heartbeat. If you know what to look for, you can start to notice the colors of the noise that make up the soundscape around us.
If you decompose a sound wave, you can break it down into two fundamental characteristics: frequency, which is how fast the waveform is vibrating per second (one hertz is one vibration per second), and amplitude (sometimes measured as “power”), or the size of the waves. The noise types are named for a loose analogy to the colors of light: White noise, for example, contains all the audible frequencies, just like white light contains all the frequencies in the visible range.
In musical sound waves, the frequencies are spaced at intervals that we find pleasing to the ear, creating a harmonic structure that gives a sound its unique tone quality, or timbre. (This is what makes the same note sound different on a flute than it does on a violin.) The noises we hear every day—boots stomping across the floor, a car honking outside, the jingling of keys—are made up of sporadic waveforms, a random distribution of frequency and amplitude.
And then, in a separate category, there are the colored noises. Unlike the inconsistent bang of a drum or shouting voice, these sounds are a continuous signal, but they aren’t exactly pleasant. The word “noise” actually comes from a Latin word for nausea; in audio engineering, the term describes any unwanted information that interferes with the desired signal, like static on the radio.
Pure white noise sounds like that hissy “shhh” that happens when the TV or radio is tuned to an unused frequency. It’s a mixture of all the frequencies humans can hear (about 20 Hz to 20 kHz), fired off randomly with equal power at each—like 20,000 different tones all playing at the same time, mixed together in a constantly changing, unpredictable sonic stew.
The other colors are similar to white noise, but with more energy concentrated at either the high or low end of the sound spectrum, which subtly changes the nature of the signal. Pink noise, for example, is like white noise with the bass cranked up. It’s a “shhh” sound with a low rumble mixed in, like the soft roar of a rainstorm.
Pink noise sounds less harsh than white noise because humans don’t hear linearly. We hear in octaves, or the doubling of a frequency band, which means we perceive as much sonic space between 30-60 Hz as between 10,000-20,000 Hz. We’re also more sensitive to higher frequencies (one to four kHz, which is about the frequency of a crying baby, sounds the loudest), so white noise, which has the same intensity at even the highest tones, can sound way too bright to our ears. The energy in pink noise drops off by half as the frequency doubles, so every octave has equal power, which sounds more balanced.