White noise, a sound that can drown out background noise and increase sleep quality, has gained popularity in recent years as many people seek improved rest and focus.
There's now growing interest in other coloured noises such as pink, brown and green noises.
Scientists say sound consists of waves, similar to light. It can be described using colours based on the distribution of its frequencies just like light appears red at lower wavelengths, while it is more blue and violet at higher wavelengths.
Thousands of people listen to these sounds on platforms like YouTube and through meditation apps.
"Many people are looking for how to use sounds to improve their sleep. Most of these applications and programs and devices use continuous sounds, which are helpful to mask external noises and allow people to sleep better by not being disturbed by other sounds around them," said Dr Roneil Malkani, an associate professor of neurology in the sleep medicine division at Northwestern University in the US.
With potential benefits for sleep, concentration and relaxation, the coloured noises provide different auditory experiences. While white noise is often described as a static-like sound, brown noise has deeper rumbling tones. Green noise sits between white and brown noise in frequency.
Pink noise may enhance memory recall
In 2019, researchers at Northwestern University looked into whether pink noise can help enhance sleep and memory.
Malkani says his team used pink noise to boost slow-wave sleep. These slow brain waves happen during a person’s deep sleep, which is pivotal for a restful night, according to the research team.
"The reason why we use pink noise is because the distribution of sound frequencies mirrors the distribution of brainwave frequencies we see on the brainwave tests on a sleep study during slow wave sleep. What we're trying to do with this research is to stimulate or boost slow-wave sleep, which is one form of deep sleep. And pink noise mirrors the types of frequencies seen in that stage of sleep," said Malkani.
Participants of the sleep study listened to short pulses of pink noise at specific times during slow waves.
Malkani explains the team exposed the sounds to participants in a way specifically intended to enhance the slow waves in sleep.
As a result, participants showed better memory recall after they slept with the pink noise sound pulses.
"We found that in older adults, we can enhance their slow waves and that they have better memory recall after sleep, when they got the sounds, versus after sleep if they did not get the sounds,” Malkani said.
Despite their popularity, the science behind these coloured noises is fairly new with only a few small studies behind it.
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