White Noise
Many night time sleep sound machines generate something known as white noise. What is this, and why is a sound named after a colour?
What Is White Noise?
White noise is a hissing sound created by combining noises of various different audible frequencies. In technical terms it has a "flat frequency spectrum": all the different sounds across a range are combined with equal intensity. The result is a noise that contains all the different sounds but in which none of them are dominant. It's called "white" noise by analogy with light, where white light contains all the different colours of the spectrum.
Why Use White Noise?
Why do people buy white noise machines? What's the point of something making a hissing sound all night?
White noise is often used to help sleep because of its combination of components and lack of pattern. All the different sounds being heard at once means that it's difficult for the brain to pick out any particular one. A disturbing external sound, such as traffic noise, can be "absorbed" into the white noise and hence also blocked out from our perception.
White noise doesn't work for all people or all sounds but can be very effective in the right circumstances.
Oh, and you may be interested to know that there are other "colours" of light as well: grey, pink and brown are the most common.