What is white noise? All you need to know about sounds named after colors

Here are some aspects of sound and sound waves, the familiar one we’ve heard of called white noise, and some of the lesser-known "colors" of sound. Whether used by an individual to relax or a sound engineer to test audio, here are colors of noise, what they sound like, and what they can be used for.

The well-heard-of white noise

White noise is a specific noise that contains all the frequencies of sound that we can hear, just like white light includes all the colors on the spectrum. Some people describe it as having a similar sound to static from a television or radio. It can also be intense and high-pitched, like a fan, air conditioner, or a vacuum. Due to its span of sound, it is sometimes called broadband noise because of the multiple bands of sound it encompasses.

It’s also a sound that some studies indicate may be good for sleep and inducing relaxation. According to some research, it can have a calming effect on humans. It is thought to help people sleep better, improve work performance and reduce crying in babies. A study from the journal Archives of Disease in Childhood demonstrated that white noise could help induce sleep in newborns.

What is white noise?

White noise is defined as noise that has a mixture of all the audible frequencies that the human ear can detect. According to Merriam-Webster dictionary, it is “a heterogeneous mixture of sound waves extending over a wide frequency range.” The frequency range is between 20 to 20,000 hertz.

White noise has a flat spectral density, meaning its amplitude does not change throughout its frequency range.

It is a combination of all the frequencies of sound played at once. White noise is made of equal energy for each frequency. It is often said to conceal the sounds of other noises because of its frequencies, leading many people to experience its calming effects. One issue with white noise, however, is that all the frequencies come through at the same intensity. Many people find high-pitched sounds to be unpleasant, so white noise can be annoying instead of relaxing for them.

Pink Noise

Pink noise also contains all the audible frequencies of sound, but the lower frequencies are amplified and the higher frequencies are diminished. In pink noise, the frequencies of the higher octaves are decreased by three decibels. It has a lower pitch than white noise.

Because the higher frequencies are less intense, some people find pink noise less overwhelming than white noise. The calming lower frequencies can make pink noise sound more soothing. It is a more even, flat sound, like a steady rain, leaves rustling in the wind, or waves on a beach

Pink noise is also thought to enhance sleep and reduce stress by covering up distracting background noises. There has been a study on the benefits of pink noise on Alzheimer’s Disease as well. According to research published in Annals of Clinical and Translational Neurology and led by researchers from Northwestern Medicine Feinberg School of Medicine, pink noise can help people with Alzheimer’s by improving their memory and cognitive function.

Brown Noise

Brown noise, also called red noise, is another type of broadband sound. It is referred to as brown noise because its change in sound signal from one moment to the next is random. It is produced by the same process that causes Brownian motion — the random movement displayed by particles that bump into one another when placed in fluids.

The noise is not named after the color itself but after Robert Brown, a botanist from Scotland who first observed the motion of pollen grains in random directions when placed in water in 1827, now referred to as Brownian motion. Although Brown first noticed the moving particles, it wasn’t until later that it began to be explained in detail. In 1905, Albert Einstein published a study on Brownian motion explaining that the pollen was being moved by the water molecules in the liquid and demonstrating the existence of atoms.

Brown noise contains noise from every octave of the sound spectrum, but the power behind the frequencies lessens as the frequency increases, so there is more power in the lower frequencies. The sound heard is deeper than white noise. It contains all the frequencies similarly to white noise, but at lower frequencies.

Brown noise is said by some to improve cognitive performance. It has a softer sound than white or pink noise. It has been likened to the sound of heavy rainfall or a thunderstorm.

Its alternate name, red noise, comes from an analogy to light, which has more lower-frequency waves than white light.

Blue noise

A lesser heard of noise on the sound spectrum is blue noise. This noise is also called azure noise. This sound has a higher frequency than white noise. It is also the opposite of brown noise due to its decibels. In brown noise, the power, and energy of the signal increase as frequency increases. In addition, each successive octave of blue noise increases by three decibels, so each octave has as much energy as the two octaves below it combined.

This bias toward higher frequencies gives blue noise a high-pitched sound lacking any bass. The name comes from visible light, where the color blue is at the higher end of the frequency spectrum.  

As mentioned in the journal Proceedings of the IEEE, blue noise is often used in a process known as dithering, which is adding noise to enhance sound quality and prevent distortion. Because of the frequency of blue noise, the sound is added to create a smooth, softer, and continual transition between sounds.

Lesser-known colors of noise

Grey noise

Grey noise is random noise that contains all frequencies, similar to white noise, but because it has more power at the top and bottom of the frequency spectrum, to a listener, every frequency sounds equally loud.

Sometimes, the sound is described as having some type of depth effect to it when people listen to it. This sound is based on each listener's perception of listening to it, so it can sound slightly different to each person. For this reason, it is often used in the testing hearing.

Violet noise

Violet noise is higher frequency than blue noise. It increases by six decibels per octave. It is also called purple noise. This sound is sometimes used to help relieve tinnitus, or ringing in the ears.

Black noise

Black noise means all frequencies have zero energy, except for a few narrow bands or spikes, so that there is effectively no sound. To correlate with the colors, just as black light is the absence, or complete absorption of visible light, black noise is the absence of sound. It is complete silence. Despite not having sound, black noise is sometimes used for meditation by using the absence of any noise and listening to absolute silence to block out other distracting noises.

Sound waves and the colors of noise

The noises mentioned here are made up of sound waves. This allows us to process the various sounds around us. The two main factors of sound waves are amplitude and frequency. Amplitude refers to the volume of sound, or how loud or soft a sound is, measured in decibels. The louder a noise is, the higher its decibel. Frequency is the vibration of the wave, measured in Hertz.

The signals in the noise don’t change at each of the frequencies, making it constant. The sound waves give noise colors, their different sounds, and pitches.

This story is part of THE SOUND GUIDE, Interesting Engineering's special issue dedicated to all things sound and audio.