You’ve probably heard people talk about white noise to describe a rushing, static-like sound. The term white noise seems like a paradox. How can a sound have a color? Isn’t sound invisible?
Yes, you are absolutely correct—sound waves traveling through air are just as invisible to the naked eye as are the air particles themselves. (In order to “see” sound, you would need to be able to see the locations and movement of the air particles.)
The origin of the term goes back to the study of optics and light. Both light and sound depend upon the science of waves. All waves have frequency—the number of cycles a wave does in one second. For light, changes in frequency are perceived as changes in color. For sound, changes in frequency are perceived as changes in pitch. Take a look at the light spectrum below and notice how the color changes as the frequency changes from left to right.
If you were paying close attention, you probably noticed that a certain color was missing from the spectrum. White is not part of this visible light spectrum. This is because white light is not composed of a single frequency. Instead, white light is the combination of all the visible colors (or frequencies) of light.
Similarly, white noise is composed of all the audible frequencies of sound. For the human ear, that means frequencies from 20 Hz to 20,000 Hz. White noise sounds like a dull roar or static. because each frequency sounds with an equal intensity.
You might hear white noise referred to as a type of broadband noise. This is because white noise is made up of a broad band of frequencies.
White noise is not the only type of broadband noise with a color name. There is also pink noise, red noise, and grey noise. Like white noise, the other types of noise have frequency content across the full range of audibility, but they do not have the equal intensity at each frequency. For example, the intensity of pink noise at a given frequency is inversely proportional to the frequency. This leads to higher intensity at low frequencies and lower intensity at high frequencies. A table of each color of noise as well as its frequency-intensity relationship is shown below:
Color of Noise
|White noise||Equal intensity across all frequencies|
|Red Noise (Brown Noise)|
|Grey Noise||Inverse A-weighted intensity at each frequency|
Red noise, like pink noise assigns higher intensities to lower frequencies. In red noise (also confusingly called Brown noise—after the scientist Robert Brown, not after the color) the intensity is proportional to the inverse of the square of the frequency. Grey noise, on the other hand, adjusts the intensity at each frequency according to the inverse A-weighting curve. This is because our ears don’t hear all frequencies equally well. Grey noise takes care of perceptual differences by boosting frequencies that human ears naturally struggle to hear so that all frequencies have the same perceived loudness, rather than having the same intensity.
Broadband noise, like white noise, can be useful in many applications. It can be used to evaluate how products, spaces, or materials respond at different frequencies. White noise is often used to mask other sounds and can be especially useful for light sleepers or for increasing the privacy of a conversation. When the ear is busy perceiving sounds at all frequencies, it has greater difficulty picking out extra individual sounds.
So, it turns out that noises don’t have colors in the visual sense of the word. However, designating noise by color is a useful tool, one that might just help you get a good night’s rest.