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, which has a harsh sound, but more often pink noise, whose power rolls off at higher frequencies, or other colors of noise.
White Noise devices are available from numerous manufacturers in many forms, for a variety of different uses including audio testing equipment, sound masking products, to sleep aid devices, and power-nap machines. The sleep-aid and nap-machine products may also produce other soothing sounds, such as music, rain, wind, and ocean waves mixed with -- or modulated by -- white noise. White noise generators are often used by sufferers of tinnitus to mask their symptoms.
Sound masking devices are often used to protect privacy by masking distant conversations, for example, in a psychiatrist's waiting room. Studies have shown that employing white noise machines in offices improves productivity by reducing distractions.
The sounds generated by digital machines are not always truly random, rather are short pre-recorded audio-tracks which continuously repeat at the end of the track.
For sound masking devices, manufacturers recommended that the volume of white noise machines be initially set at a comfortable level, even if it doesn't provide the desired level of privacy. As the ear becomes accustomed to the new sound and learns to tune it out, the volume can be gradually increased to increase privacy.
For sleeping aids and power-napping, manufacturer's recommend the volume level be set slightly louder than normal music listening level, but always within a comfortable listening range.
Most modern white noise generators are electronic, usually generating the sound in real-time within audio test equipment, or via electronic playback of a digital audio recording.
Simple mechanical machines consist of a very basic setup, involving an enclosed fan and (optionally) a speed switch. This fan drives air through small slots in the machine's casing, producing the desired sound.
Canadian Research Council study on white noise use in offices PDF (451 KiB)