True white noise is the electronic version of an enormous band playing every single note (and intervals between notes) at the same volume, at the same time. It’s technically defined as the even distribution of all sound frequencies within a normal audible range, just like white light is an even distribution of all colour wavelengths within a normal visual range.
Fans, crackling campfire sounds, and rainstorms—that is, sounds you’re likely to hear from one of the many "white noise" apps available on your smartphone—are not true white noise. Rather, they’re colloquially referred to as "white noise" because they are types of consistent background noise. Genuine white noise is actually rather irritating to most people, since it contains a lot of high-frequency sound.
Both true white noise and other types of ambient background noises help some people sleep thanks a principle known as sound masking. If you’re in a dark room and somebody shines a flashlight at you, your attention will be drawn to the source of light right away. However, if someone shines a light your way in broad daylight, you may not even notice it.
The same principle applies for white noise. Even though you’re not consciously aware of it, your hearing still works while you’re asleep, if not to the same degree as in your waking life. With the help of ambient noise that covers a wide range of frequencies, you become more resistant to sudden errant noise in your environment.
That being said, if you’re not a particularly light sleeper, or if you live in a quiet environment, white noise is probably not necessary. One particular point of caution is important for parents of newborn babies to note. Even though white noise can be soothing to infants, some doctors believe that early, sustained exposure to white noise can have a damaging effect on hearing. Make sure to check with your paediatrician before introducing white noise into your child’s sleep routine.