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Waves crash. Tree branches vibrate. Mosquitoes hum. These phenomena take place every day but not everyone necessarily hears them. Indeed, some sounds happen in places that are hard for humans to access or below the level of what we can perceive. But more and more, scientists are listening in.

From a root system up to 90 feet underground, to balloons floating 70,000 feet above earth, a series of recent audible insights are being showcased this week by experts gathering at the annual Acoustical Society of America conference. The new data revealed by these sounds are helping scientists better understand the natural world.

For example, last year researchers discovered that climate change is speeding up the transmission of sound in the ocean, with potential implications to marine life. More recently, in March, a study published in Nature revealed how the pandemic reduced shipping noise in the deep-ocean, lessening the amount anthropogenic disruption to the marine environment’s own soundscape upon which animals rely. And this summer, ecologists plan to monitor how remote Alaskan wildlife is responding to climate change and industry; they’ll do this by listening to birds chirping and bugs buzzing to analyze their abundance and activity patterns.

It’s a “chance to really listen in on a new world,” says Daniel Bowman, a senior scientist at Sandia National Laboratories, a Department of Energy national security and technology lab, adding that this type of research provides “another perspective on sound from a different vantage point”—in his case, miles above the ground.

A stratospheric symphony

In a layer of our atmosphere far above where commercial planes soar, a group of scientists, including Bowman, have been releasing balloons to record otherwise un-heard sounds from the relatively calm region of our sky. Using low-frequency microbarometers they were able to detect what the earth sounds like from so high up—from thunderstorms to wind turbines and freight trains.

In a 19-day-long recording showcased at the conference, a calm, wind-like whooshing can be heard interspersed with crackles and pulses. That whooshing is the sound of waves crashing into each other in the middle of the ocean. “You’re listening to the ocean below, basically slamming into itself, and making the signal that’s traveling up and it’s going past the balloon and actually heating up the upper atmosphere when the sound wave dissipates,” explains Bowman. In other words, the recording measures how the ocean changes energy patterns 60 miles up into the sky. “There’s all this really amazing stuff that goes on,” he says. “It never really occurred to me it was happening, and then with this new vantage point we can actually directly measure it.”

Researchs inflate a solar hot air balloon equipped with with an infrasound microbarometer. (Darielle Dexheimer—Sandia National Laboratories)
Researchs inflate a solar hot air balloon equipped with with an infrasound microbarometer.
Darielle Dexheimer—Sandia National Laboratories

Bowman hopes to use his findings to inspire others—whether a middle school science student or professional researchers—to conduct similar experiments to better study this little explored region of our sky. The balloons his project used are cheap, using $50-worth of materials, and easily constructed, and can be used not only for recording acoustics but for things such as aerial photography or measuring greenhouse gasses. (His experiment is also part of a proof-of-concept with NASA to explore seismic activity on Venus.)

“It’s not like we’ve explored every nook and cranny of the stratosphere,” he says, “so it still has prizes for us.”

The sound of tree roots

There may also be lots new to discover underground.

The world’s largest tree by weight and landmass lives in southern Utah. Known as Pando, the 9,000-year-old aspen grove is a singular organism made up of more than 47,000 genetically identical trees. As a conference presentation detailed, the nonprofit Friends of Pando used hydrophones, underwater microphones, to record what’s thought to be vibrations being passed along Pando’s root system during a thunderstorm.

“It’s similar to two cans connected by a string. Except there are 47,000 cans connected by a huge root system,” Jeff Rice, a sound artist working with the nonprofit, said in a statement. Leaves rustling in the wind caused the hydrophone to pick up underground signals—the more vigorously the leaves and branches shook, the stronger the signal.

“While it started as art, we see enormous potential for use in science,” said Lance Oditt, the nonprofit’s executive director. “Wind, converted to vibration (sound) and traveling the root system, could also reveal the inner workings of Pando’s vast hidden hydraulic system in a nondestructive manner.” The group hopes to use data from these recordings to study water movement, insect colonies, and root depth, among other things.

That said, artistic interpretation of nature’s sounds is, in itself, another valuable way of making sense of the world we live in. Also showcased this week at the conference, for example, was a percussion-based song interpreting 120 years of ocean oil drilling. And the trend extends outside the conference’s walls. An album by U.K. musician Cosmo Sheldrake released last month uses a cacophony of underwater recordings as the basis of its songs—from singing humpback whales and spluttering coral reefs to a rare recording of Britain’s last remaining killer whales threatened by chemical contamination. Meanwhile, a group of University of California Berkeley grad students in 2018 transformed over a century of rising temperatures into an original musical composition.

Sound is a powerful sense. Rainfall and lapping waves can calm us. Sirens send us warnings. And just a snippet of a song’s chorus can bring back a lost memory. By unearthing previously inaccessible sounds scientists are now connecting us more intimately than ever to the world around us. And by turning data into music, artists can help us experience potentially abstract science in a visceral new way. Together, this can force us to think, and feel, differently about nature’s wonders, and our impact on the planet.

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