Daniel Bowman sends cell phones into the stratosphere aboard solar-powered hot-air balloons. It’s amazing what they can record.
These balloons listen to the Earth from a dozen miles up, but not in frequencies that the human ear can detect. The balloons are recording in infrasound, which is below human hearing. Scientists have captured sounds of windmills, earthquakes, cities swarming, and mysterious crackling.
“In our infrasound realm, the planet has a lot to offer,” Bowman told Insider. He is a Sandia National Laboratories scientist. “I think one of the reasons humans can’t hear infrasound is because we’d go nuts if we could.”
Take the sound of the ocean, which you can hear alongside the mystery sound in the recording below. This is 19 days of infrasound recordings, sped up by 4,400 times.
The first thing you hear is the sound of ocean waves crashing, far below. That’s the sighing sound in the background. It would sound like watching television if you heard it with your ears. And it would last for days.
Then there’s a flurry of crackling and rustling. That’s the mystery sound.
It could be turbulence, distant thunderstorms, or meteors burning up as they plummet through the atmosphere. According to Bowman, this noise also appeared in the last infrasound balloon program in the 1960s.
“We’ve been picking it up 50 years ago and we’re still picking it up, still don’t know what it is,” Bowman said.
Almost every flight picks up unidentified sounds, according to Bowman.
“It’s not like there’s something inherently mysterious or really profoundly different up there. It’s more due to our own ignorance,” he said.
The world of infrasound is vast and largely unmapped. The stratosphere, which is located in the lower part of the atmosphere where sound bends up, makes it the ideal place to listen for hidden sounds from the earth, even if they are thousands of miles apart.
Bowman started using these balloons to listen for the rumblings of volcanoes, hoping to contribute to understanding them better and maybe even improving early-warning systems for eruptions. But it turns out there’s even more to hear in the stratosphere.
Anyone can fly an eavesdropping balloon
Big atmospheric balloons may have gotten a bad rap after the Pentagon shot down at least one of them on suspicion that it was a Chinese spy balloon.
But Bowman wants to see more people flying infrasound-recording stratosphere balloons.
“This is something that a group of middle schoolers could do,” he said. “The more eyes in the sky, or ears in the sky we get, the more we learn.”
He estimates that his research team has flown 100 of these balloons, each one made of just $50 of materials that you can buy at a hardware store.
Weather balloons are often designed to rise up and up and up, gathering data all the way, until they pop, then parachute to the ground. Bowman’s balloons must be in the stratosphere to record sound. They also need to move at the speed of the wind, so it doesn’t roar past and drown out the faint noises he wants to catch.
That’s why he builds his hot-air balloons out of dark material that absorbs energy from the sun, but only enough to rise to the stratosphere and stay there.
“Our balloons are basically giant plastic bags with some charcoal dust on the inside to make them dark. We build them using painter’s plastic from the hardware store, shipping tape, and charcoal powder from pyrotechnic supply stores. When the sun shines on the dark balloons, the air inside heats up and becomes buoyant,” he explained in a press release.
The balloon will cool down and come back to earth at sunset.
“You don’t have to worry about balloons that just keep on going, off into places they shouldn’t go,” Bowman said.
Bowman believes that anyone could build a similar balloon using an old phone, the RedVox application, and launching it in the stratosphere.
Just don’t forget one crucial step:
“In the current environment, I highly recommend reaching out to the [Federal Aviation Administration] and notifying the FAA of your intent to launch a balloon, so that they know what it is and so that it is not misinterpreted,” he said.
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