White noise is: A sound containing a blend of all the audible frequencies distributed equally over the range of the frequency band. White noise is analogous to white light which contains all the colors of the rainbow together. The term "white noise" as used in this article will be applied a little looser to include a host of sounds (waterfall, rain and brook as examples) used for blocking out unwanted sounds.
Because white noise contains most sound frequencies, it is commonly used to mask other sounds. If you are in a hotel and voices from the room next-door are leaking into your room, you might turn on a fan to drown out the voices. The fan produces a good approximation of white noise. How does this "white" sound work and why does it mask out other noise?
Here is one way to think about it. Let's say two people are talking at the same time. Your brain can normally pick out one of the two voices and actually listen to it and understand it. If three people are talking simultaneously, your brain can probably still pick out one voice. However, if 1,000 people are talking simultaneously, there is no way that your brain can pick out one voice. It turns out that 1,000 people talking together sounds a lot like white noise. So when you turn on a fan to create white noise, you are essentially creating a source of 1,000 voices. The voice next-door makes it 1,001 voices, and your brain can't pick it out anymore. One thing to keep in mind when using white noise is that more is not always better - it is usually best to use just enough to help mask the offending noise, not completely overwhelm it.
There are three sources of white noise that are commonly used to help mask out annoying sounds: mechanically generated, electronically generated and recordings usually on CD format.
The mechanical devices produce a sound very close to that of a fan. The Sleepmate by Marpac is this type of device and has been around for a long time. This is the most popular type of white noise machine; probably because it produces a sound that we are all familiar with and don't find unpleasant. One drawback of the mechanical units is that they are limited in volume. Since they produce the sound mechanically, it isn't possible to simply turn up the volume.
White noise is commonly generated with a digital or electronic sound synthesizing device. Sound designers, with some processing and filtering, can create a multitude of effects such as wind, rain, waterfall or surf. The main consideration when choosing a digital device is if the device loops a recorded clip of sound or synthesizes its own sound. By looping we refer to a few seconds of sound that is digitally recorded and played over and over. They try to match up the end with the beginning but if you listen carefully you can detect where the segment begins - this is irritating to some people. The better digital sound devices actually synthesize the sound instead of playing a recording, therefore no looping.
Compact discs are a popular and inexpensive way (not counting the cost of a stereo system) to bring white noise into your home or office. You can get almost any sound imaginable but the most commonly used are nature sounds like rain, thunder or babbling brooks and nature sounds overlaid with soft music. The advantages of CDs are a high quality sound and low cost. The disadvantage is that, if you are using the sound to help you sleep, the interruption when changing tracks or cycling will often times wake you up. Generally the best uses for CDs are for office privacy, meditation or just relaxing.
Michael Kirtley is owner-manager of Nature's Tapestry ( http://www.naturestapestry.com ), a site with information about how to choose a white noise machine.