Turn that radio down, it’s too bright!

Noise comes in many colors. We’re all familiar with white noise–the static on the radio and TV–but unless you work with electronics, you may not have heard about other colors of noise.

Contrary to what you might think, meaningless random noise is not always the same. Different kinds of noise make different sounds and images, so to keep track of the different types they are named after colors, typically in reference to light with a similar spectra. For example, the frequency spectrum of white noise is relatively flat–you’re likely to find the same amount of noise at all frequencies. White light contains equal amounts of every color and also has a flat spectrum. Essentially, if we could translate the flat, even noise into color, it would be white.

If noise is essentially meaningless, why do we care about distinguishing different kinds? Well, as Dan Ellis, a professor at Columbia University, says, “The technical challenges in, say, building a cell phone system, or an ultrasound machine, come from trying to extract a desired information ‘signal’ from a background of unwanted ‘noise’.” Each color of noise has a certain post-processing method to decrease or remove the signal from the actual, desired data. But sometimes random noise is useful. White noise can block out other sounds which helps some people–especially those in crowded cities–get a good nights rest.

The other colors of noise are not flat, but concentrated at specific frequencies. Blue noise, for example, increases with increasing frequency. This is useful in graphics engineering to trick the eye into seeing a continuous color gradient instead of the actual quantized pixels. In audio, blue noise is more annoying than useful because of the high-frequency hiss. Pink noise is the exact opposite, it decreases with increasing frequency. Since pink noise is concentrated at the low end of the spectra, it’s often used in auto to increase the bass and cut down on high-frequency notes. On its own, pink noise sounds like the low hum you hear on an airplane.

Brown noise–confusingly also known as red noise or more comically “drunkard’s walk” noise–lies between pink and white noise. It’s called brown not in reference to the color, but because it can be generated by mimicking Brownian motion. Brown noise is a more pleasant version of white noise with lower-frequency notes reminiscent of waves on the beach or a windy day.

There are also several other unofficial colors–orange, black and green–but white, blue, pink and brown are the most useful. So next time you’re assailed by a blast of static or mechanical rumbling think of what color you’re hearing. Maybe you’ll finally get some sleep on that next red-eye flight back home.

 

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