A. Like all complex sounds, white noise is a mixture of many frequencies: high, low and in between. What sets it apart, though, is that all these frequencies are roughly equal in intensity, giving rise to a toneless hiss rather than something recognizable.

Many people swear by white noise as a sleep aid, but "there's really no clear answer" to why it's soothing, says Bob Lutfi, a UW-Madison expert in human perception of sound. That's because a whole host of sounds share characteristics of white noise, and people undoubtedly differ widely in what they consider calming.

That said, Lutfi guesses some people find comfort in white noise because it approximates certain peaceful sounds in nature, like ocean waves or rustling leaves. White noise also does a great job of blocking unwanted sounds, such as harsh mechanical noise on the factory floor.

Then again, other forms of white noise can be totally annoying. Take, for example, the sound a TV makes when it stops broadcasting and the screen fills with static. "That's visual white noise, and the sound you're listening to is similar to white noise," Lutfi.

Incidentally, the term "white noise" comes from white light. Just as sound is made up of many frequencies, light contains many different wavelengths. And in white light these wavelengths all share the same intensity so that no single color stands out.

-- Produced in cooperation with University Communications

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