A white noise machine, sometimes also called a sound machine or sound conditioner, is a device that produces a stream of background noise - typically the type of sound that mathematically qualifies as "white noise", though sometimes these machines actually produce pink noise or brown noise instead. They're designed to help people sleep better and focus by drowning out distracting sounds. They can be especially helpful for people with tinnitus, since they drown out the distracting buzzing or ringing sound the disease causes.

There are two types of white noise machines: those that play a loop of white noise sound, and those that generate sound. 

Also worth noting are white noise apps. These can be useful for those on a tight budget, as many of them are free or very cheap, but the noise that comes from tiny phone speakers isn't typically very high-quality, and lacks many of the benefits of a purpose-built white noise machine.

While some people don't mind them, white noise machines or apps that play sound on a loop can become annoying over time, even if the cut between loops isn't jarring (which it sometimes is). Therefore, most high-quality sound machines play non-looping white noise.

A subset of non-looping white noise is actual fan sound, as is found in the Dohm, which has a real fan inside. This creates a more soothing sound than a box fan, with heavily-adjustable volume and pitch. As it isn't pre-recorded, you avoid the problems that come with recorded or looped sound.

Why does white noise work as a sleep aid? The answer lies in creating a consistent sound environment. 

Distracting sounds, like a car horn or a snoring spouse, can prevent you from getting to sleep and staying asleep. A white noise machine uses sound masking to keep these noises from interrupting your sleep. Many people - from light sleepers to people living in busy cities to ICU patients - have a much better chance of getting a good night's sleep with a white noise machine. 

White noise can also help your baby sleep, and many sound machines are made specifically for babies. They hang from cribs, have night lights, and contain other baby-friendly features like child locks.

The best part about this is that people have developed no reliance on white noise. Getting a white noise machine seems to be similar to any other sleep hygiene improvement, such as getting a better mattress: it improves your sleep quality, but doesn't mean you won't ever be able to sleep again without it.