By Dani Blum and Illustrations by Alexis Jamet Sept. 23, 2022

The noise sounds like wind, or heavy rain, or the steady hum of an airline jet. It sounds like water rushing somewhere in the distance, like a gentle fan ruffling currents of cool air. It’s soothing, steady, slightly rumbly.

Welcome to the cult of brown noise , a sometimes hazily-defined category of neutral, dense sound that contains every frequency our ears can detect. Brown noise is like white noise but has a lower, deeper quality. It gained a fervent following over the summer, picking up speed in online A.D.H.D. communities, where people made videos of their reactions to hearing it for the first time. Many said it allowed their brains to feel calm, freed from an internal monologue. Some invited their viewers to try it too, and commenters chimed in, claiming that brown noise was not only a tool to help them focus, but could relieve stress and soothe them to sleep.

On YouTube, eight-, 10- and 12-hour-long brown noise videos have been viewed millions of times; there are also curated Spotify playlists dedicated to the sound and specialized apps to play it.

“It’s definitely a trend going on,” said Dr. Shivnaveen Bains, a child and adolescent psychiatrist at Cleveland Clinic Akron General who treats people with A.D.H.D. She added that many patients have come in asking about brown and white noise as of late, often pointing to posts they’ve seen on social media.

Over the last 20 years, scientists have uncovered evidence that immersive sounds like white, brown and pink noise may help the brain to focus, sleep or relax — especially for people with A.D.H.D. But experts say it’s hard to delineate and categorize sound, so the differences in effect among the various colors are not always well established. Many sound similar or bleed into each other, said Daniel Berlau, a professor at the Regis University School of Pharmacy who has studied the impact of white noise on A.D.H.D. “It’s not as scientific as people would think,” he said.

Turn up the volume to get a feel for how your own ears and brain react. Here’s what we know about how these noises might influence the ways we focus, sleep and relax.

The human ear only detects sounds within a certain range of frequencies — those that stimulate the cochlea, a spiral-shaped cavity swirling in our inner ears. When we play brown noise, we hear every frequency that the ear is able to detect at once, said Dr. Berlau. Listening to all of these frequencies at once creates an immersive, even smothering, experience that some people may find pleasant.

Researchers have identified a spectrum of sounds that have been named after colors; each is defined by the relative intensity of different frequencies of sound. (The origins of each color’s name aren’t definitively known.) For people with standard hearing function, brown noise’s better-known and better-studied cousin, white noise, has a more hissing sound than brown noise. Pink noise is a softer version of white noise, playing lower frequencies a bit louder. Violet noise plays higher frequency sounds louder than brown noise and also makes a hissing sound. Gray noise, which is calibrated so your ears hear all frequencies at the same volume, sounds similar to white noise, but is smoother. White, brown and pink noise are the most commonly known and studied, because scientists have the clearest consensus on how to replicate them.

Brown noise’s name comes from Robert Brown, a Scottish botanist, who discovered “Brownian motion” — the way pollen grains suspended in water “dance” under a microscope. Brown noise, experts say, mimics that movement, with sound signals that change at random, from one moment to the next, producing static.

As for whether brown noise is more effective for focus or sleep than any of the other types of this immersive noise, the answer is no, said Yamalis Diaz, an assistant professor in the department of child & adolescent psychiatry at N.Y.U. Langone Health, who specializes in A.D.H.D. “No research suggests a specific kind of noise is the key. Frequencies can activate and stimulate the brain in different ways.”

White noise sounds like static. Just like white light emits all pigments of visible light at an equal intensity, white noise mixes sounds of every audible frequency. It has a more fizzling sound than brown noise.

Pink noise is a sanded-down version of white noise, playing lower frequencies a bit louder. It sounds like gentle rainfall, or the light sputter of a sprinkler, with less hissing than white noise.

Brown noise contains all frequencies, like white noise, but plays the low frequencies at a louder level and the high frequencies on a softer level.

There’s a popular theory called stochastic resonance, in which the presence of white noise allows the brain to tune into hard-to-hear tones — in music, people’s voices, or ambient sounds — that otherwise go unnoticed, said Göran Söderlund, a special education professor in Sweden who also studies neuroscience and cognition. In other words, according to the theory, just by turning on white noise, you can hear and focus on other stimuli more clearly.

Both internal and external cues can distract the brain, Dr. Diaz said. “You can be completely overwhelmed in your own brain by thoughts like, ‘Did I turn off the curling iron?’” she said. Noise that is just stimulating enough to activate the brain but is not overwhelming can help drown out some of that internal chatter, she said, as well as mask background noise.

There is not robust research to suggest that brown noise alleviates stress. “Some people think anxiety might be quelled by having a noise blanket to filter out the sounds,” Dr. Berlau said. But for some, a constant blare of noise could be distracting or provoke more anxiety.

“It’s not like everyone agrees this always works,” he added. There is also the power of the placebo effect, especially as people seek out brown noise already convinced that the sound can soothe them.

Dr. Soderlund and other researchers have studied the benefits of white noise for children with reading disabilities and A.D.H.D. In one experiment, children with reading disabilities completed a 30-minute test that involved reading and remembering words; those who listened to white noise through headphones generally performed better at the test.

The noise enables them to better concentrate, Dr. Soderlund said, and to complete academic tasks.

That may be because, in people with A.D.H.D., the prefrontal cortex of the brain might struggle to filter out the stimuli a person encounters in any given environment, like chatter from a nearby conversation or an image flashing across someone else’s screen, said Dr. Bains.

Those with A.D.H.D. may not have enough dopamine in their brains, a chemical that impacts attention and motivation, Dr. Diaz said. Without enough dopamine, the brain stays “hungry” while you’re trying to concentrate, Dr. Diaz explained. “While one part of the brain is trying to focus, the other part of your brain is looking for food.” When you listen to a sound like brown, pink or white noise, “you’re almost assigning the circuits a task,” she said. “‘You listen to this, while I focus on this task.’”

Scientists have reached conflicting conclusions on whether any particular type of noise can help you sleep better. A 2020 review of 38 studies found limited evidence that white noise can improve sleep, despite the prevalence of white noise machines marketed for sounder nights. Some companies promote white noise machines to help babies sleep, claiming the sound mimics the environment in the womb.

There have been few studies on using brown noise as a sleep aid, though one of the claims floating around TikTok is that it can help you nod off.

A decade ago, a group of researchers conducted a small study, asking 40 participants to listen to a steady stream of pink noise while they slept throughout the night. By looking at the participants’ brain waves, the researchers saw that those who listened to pink noise had deeper sleep, with fewer complex brain waves and better responses to sleep disruptions compared to when they slept without the noise.

Dr. Berlau pointed out a simple theory for why people say noise begets sleep — be it pink, white or any shade. Sounds may block out your downstairs neighbor, the traffic and your partner’s snoring.

And, experts said, if any form of noise therapy works for you, there’s no harm in using it.

There isn’t likely to be any danger in listening to brown noise for, say, eight hours at a time, Dr. Berlau said, unless someone plays the sound at unsafe volumes (listening to noise above 70 decibels over a long period of time can damage your hearing).

Meanwhile, there are those who cherish the noise.

“If you find that happy place — a calm, quiet, consistent brain,” Dr. Diaz said, “it feels so blissful.”

Audio production by Sara Curtis. Art direction by Deanna Donegan. Design and Development by Hang Do Thi Duc.