What is white noise, as used in white noise sound machines? What it is and how it works.
What Is White Noise?
Just as we use colors to help describe different kinds of light, we can use colors to help describe different kinds of noise. White light is an even distribution of all wavelengths of light within the range of normal vision. Analogously, white noise is an even distribution of all sound frequencies within the range of normal hearing.
White noise sounds like a hiss, like the spray from an aerosol can. Although all the audible frequencies are equally represented, it sounds higher-pitched to our ears, in part because our perception of pitch is not linear, and also because our ears are more sensitive to higher frequencies.
Hear white noise:
How Does it Work?
In applications where we use white noise machines or recordings to help us sleep or concentrate, we say that it "masks" or "drowns out" other sounds. How does that work?
One of the things that makes noise annoying is that the mind notices changes in the backdrop of sound in the environment. No doubt this ability helped keep our ancestors alive, by alerting them to the faint sound made by a stealthy predator. To us, however, becoming alert to every tiny noise can be a problem, when it keeps us awake or keeps us from being able to concentrate.
White noise changes that background sound: the overall level of sound in the environment is higher, which raises the volume threshold over which noises must pass in order to be noticed. Also, the sound quality is featureless, so there are no details for the mind to latch onto and be alerted by. That is why it has a soothing quality — it allows the mind to relax.
What about Other Colors?
Because of the "hissy" quality that true white noise has, many "white noise" generators actually produce other types of sound, which serve equally well for masking noise, and sound more pleasant to most people. A few of these have colors associated with them, by extension of the analogy to light.
The most commonly used alternative to true white noise is pink noise, which is white noise that is filtered to systematically reduce the intensity as the frequency increases. Instead of having equal intensity across frequencies, as white noise does, it has equal intensity across octaves. This corrects for the non-linearity in human hearing, so that it sounds to our ears as though it is evenly spread across all frequencies. It sounds fuller or richer than white noise, more like the roar of a waterfall. Since it has higher energy in the lower frequencies, like red light, it is called "pink" noise. Many processes in nature follow a "pink noise" algorithm.
Hear pink noise:
Other interesting noise colors:
Red noise (also called Brownian noise, or "brown noise" for short, having nothing to do with the color brown!):