A white noise machine is a device that produces a sound that is random in character, which sounds like a rushing waterfall or wind blowing through trees. Often such devices do not produce actual white noise

White noise

White noise is a random signal with a flat power spectral density. In other words, the signal contains equal power within a fixed bandwidth at any center frequency...

, which has a harsh sound, but more often pink noise

Pink noise

Pink noise or 1/ƒ noise is a signal or process with a frequency spectrum such that the power spectral density is inversely proportional to the frequency. In pink noise, each octave carries an equal amount of noise power...

, whose power rolls off at higher frequencies, or other colors of noise

Colors of noise

While noise is by definition derived from a random signal, it can have different characteristic statistical properties corresponding to different mappings from a source of randomness to the concrete noise. Spectral density is such a property, which can be used to distinguish different types of noise...

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White noise devices are available from numerous manufacturers in many forms, for a variety of different uses, including audio testing equipment, sound masking products, sleep

Sleep

Sleep is a naturally recurring state characterized by reduced or absent consciousness, relatively suspended sensory activity, and inactivity of nearly all voluntary muscles. It is distinguished from quiet wakefulness by a decreased ability to react to stimuli, and is more easily reversible than...

-aid devices, and power-nap machines.