White noise is the noise created when all audible frequencies of sound are combined at the same time and the same density. It is similar in nature to white light, which occurs when all the colors of the spectrum are brought together. White noise incorporates over 20,000 frequencies. Also called white sound, white noise is used primarily as a sleep or relaxation aid.

The actual sound produced by white noise is comparable to rainfall or ocean waves. It is a gentle tone that can be found in nature. Because white noise combines all frequencies of sound ranging from very low tones to high pitches, it is very useful in masking other noises and sounds.

Fans and radio stations with no broadcast are often used as white noise simulators, providing a soothing sound that blocks out other background noises. While these methods do not produce true white noise, their effects are a good example of practical uses for white sound. The main drawback to using this variety of white noise, however, is that the user has no control over the volume or frequencies produced in this manner.

A pure white noise signal can only be produced by generators, commercially available as "white noise machines". While expensive and rather large and cumbersome, these machines are helpful for those with sleep disorders or inner ear problems, infants who have trouble falling asleep or staying asleep, and those who wish to counteract stress. White noise cancels out background noise and produces a soothing, relaxing sound. Students and those who work in the creative arts use white noise to block out external sounds that could be distracting. It also seems to have some benefit to children and adults diagnosed with Attention Deficit Disorder and its related conditions, as the masking effect of the white noise allows for better concentration.

Consumers can also purchase white noise that has been recorded onto a CD, which is less expensive than investing in a white noise machine. In addition to cost effectiveness, CDs allow the user to alter the bass and treble of the noise to mask lower or higher background sounds in any given location. Recorded media that feature sounds of nature work in much the same way. Other uses for white noise include frequency testing for electronic equipment, testing for architectural acoustics, and use as sensory deprivation aids. Musicians use white noise in electronic music to simulate higher or lower percussion sounds than those available through traditional instruments.