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You're probably familiar with white noise generators, which insomniacs often find useful as a sleep aid. White noise is a blend of random frequencies with a flat spectrum — any frequency band has the same amount of power as any other. I find white noise to be sharp and harsh. Here's an example of white noise (warning — it's loud).
Most white noise generators don't actually play white noise — they play a "colored noise" that's more soothing. Colored noises have a blend of random frequencies but some frequencies play at a higher volume than other frequencies. This gives the noise a "color" or distinctive tone. Here are samples of some common colored sounds:
- Pink noise differs from white noise — every octave contains the same amount of energy, rather than every frequency band like in white noise. It's not as sharp as white noise. It's richer, deeper. It sounds like a rushing river.
- Brown noise's power drops as the frequency increases. It sounds like pink noise off in the distance. It's lush and chocolately. I find it to be the most pleasant of all colored noises.
- Blue noise's signal power increases with higher frequencies. It sounds like air escaping from an inflated tire. It's startling rather than soothing.
- Violet noise is like blue noise but the power increases more sharply as the frequency increases. It sounds like a thinner version of blue noise.
- Grey noise is designed to fit a psychoacoustic equal loudness curve "giving the listener the perception that it is equally loud at all frequencies." I could probably fall asleep to grey noise, but it's not as velvety as brown noise.
- Black noise is silence. Here's 10 hours of it.
Wikipedia has a good article about the colors of noise with examples for each one.