The Many Different Kinds Of Noise

By Debra Kelly on Sunday, January 18, 2015
“Could we not imagine that noise [. . .] is itself nothing more than the sum of a multitude of different sounds which are being heard simultaneously?” —Jean-Jacques Rousseau (1767)

In A Nutshell

We’re familiar with white noise, and some of us even rely on it to get to sleep at night. While that’s noise that includes all frequencies, other kinds of noise are defined by the relationship between frequency and power. Pink noise had greater power at lower frequencies, while blue noise has greater power at higher frequencies. Brown noise is an extreme version of pink noise, and grey noise is stronger at the high and low ends of frequency.

The Whole Bushel

Too much of it can be annoying, too little of it can be unsettling, and it never seems to be loud enough or quiet enough. There’s a lot more to noise than we usually think about, and there are quite a few different types of it.

If you think of white noise, chances are your first thoughts are of background noise—static on a television set, maybe, or even some nondescript, generic music playing in the background while you’re trying to concentrate on something else. There’s an actual definition for white noise, though, and it’s noise that is essentially made up of all audible frequencies. That’s why it’s so good at canceling out other noise (especially soft environmental noises or ringing in the ears) and why it’s used to help people sleep. White noise is also often included in alarms and sirens, because of its ability to mask other noises and trigger the brain’s attention.

Incredibly similar to white noise is the lesser-known pink noise. Like white noise, pink noise is made up of all audible frequencies; unlike white noise, however, pink noise has a greater power in the lower range of frequencies, basically meaning that lower frequencies are louder. You hear pink noise every time you listen to the buzz of traffic—it’s also the same kind of noise that’s given off by the human heartbeat. Because of its all-encompassing frequencies, it’s often used in situations similar to white noise, masking other distracting background noises.

Blue noise is the other end of pink noise; instead of a low hum, it’s often heard as a high-pitched hissing sound. For this kind of noise, the power goes down as the frequency gets higher.

There’s another kind of noise, called either “red noise” for the color or “brown noise” for the scientist who discovered Brownian motion. Like pink noise, as the frequency increases the power of the noise decreases, but at a much greater rate than found in pink noise. To the human ear, brown noise sounds like an incredibly bass-heavy version of white noise. Reverse that power-frequency relationship and you get violet noise.

There’s also grey noise, which is noise that has greater power in extremely high and extremely low frequencies, but it’s not as strong in the middle. That’s the kind of noise that’s often used in hearing tests, because it allows doctors to determine more accurately what part of the noise spectrum the person is having trouble detecting.

Those types of noise have very strict definitions rooted in mathematical properties, but there are a couple of other kinds of noise that are less specific. Black noise is the absence of noise, but it can also be defined as white noise that exists below 20,000 Hz. Green noise is a bit that’s in the middle of the audible white noise spectrum, and orange noise is noise that has a couple of bands missing, giving it a distinctly wrong sound.

All the different types of noise can have some pretty surprising ill effects on the human body. We’re probably familiar with the idea that prolonged exposure to loud noises can cause hearing loss and the noise of a city (or an inconsiderate neighbor) can lead to sleep difficulties, but there are a few more that are becoming increasingly well documented.

In 1979, a study done around London’s Heathrow airport found a correlation between proximity to the airport and instances of admittance to the area’s psychiatric hospital. Similarly, the presence of aircraft noise has also been linked to lower test scores and reading ability in children. Areas with sudden, random occurrences of noise were linked to changes in heart rate and blood pressure, and, sadly, there’ are a number of instances where a change in the noise levels of the environment have been linked, ultimately, to suicide.

Show Me The Proof

Cause and Effects of Noise Pollution
Wired: White, pink, blue and violet: The colours of noise
LiveScience: Brown Noise, Blue Noise, White Noise, Pink Noise