Up, up in the air … scientific solar-powered balloons have recorded some strange sounds in the stratosphere researchers cannot identify.
Daniel Bowman and other researchers at Sandia National Laboratories, have been sending solar-powered balloons with sensors into the stratosphere to record sounds in the second layer of the Earth’s atmosphere, which starts at about 6.2 miles above ground.
In addition to the noises they expected to hear, they heard something they could not identify, Bowman said in research presented this week at the Acoustical Society of America’s annual meeting, held in Chicago.
“There are mysterious infrasound signals that occur a few times per hour on some flights, but the source of these is completely unknown,” said Bowman.
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How do researchers record sounds in the stratosphere?
Bowman, a geophysicist, and the other researchers, build solar-powered balloons measuring more than 20 feet across. He described them as “basically giant plastic bags,” in a press release about the team’s findings.
“We build them using painter’s plastic from the hardware store, shipping tape, and charcoal powder (put inside the balloon to make them dark) from pyrotechnic supply stores,” Bowman said. “When the sun shines on the dark balloons, the air inside heats up and becomes buoyant.”
Each balloon “only needs about $50 worth of materials and can be built in a basketball court,” he said. And they can soar up to 12 miles above the Earth.
On board are microbarometers, which detect and record infrasound – low-frequency noise or sound below the human hearing range, and GPS sensors, so the balloon can be tracked. “Right now we need to collect the balloons and the equipment once they come down to Earth,” Bowman told USA TODAY. “It’s a little bit like the ‘geocaching’ hobby, where we have to travel to wherever the balloons land and pick them up.”
The balloons were originally designed to investigate sounds made by active volcanoes. But Bowman and others began exploring the stratosphere because “the soundscape of the stratosphere was entirely unknown,” he said.
Researchers captured some mysterious sounds in the stratosphere. What is it?
Up above where commercial airplanes fly, the solar-powered balloon’s microphones have captured natural sounds such as ocean waves colliding and thunder, man-made sounds including explosions and wind turbines – and other sounds below the human hearing range.
Bowman sped up those sounds by 4,400 times to make them audible, crunching “about 19 days of data into 4 minutes,” he said. The result reveals a “sighing sound,” which Bowman said “is from distant ocean waves crashing together.”
But there’s some “rustling sounds” that researchers can’t identify yet. “We think the unknown sounds are related to emissions from atmospheric turbulence, but that is just a hypothesis at this point,” Bowman said.
A colleague at Sandia National Laboratories, Sarah Albert, who is also a geophysicist, has been investigating the possibility of a “sound channel,” which could conduct sounds across long distances in the stratosphere. She has documented the stratospheric recording of a rocket launch from nearly 250 miles away.
“It may be that sound gets trapped in the channel and echoes around until it’s completely garbled,” Bowman told CNN. “But whether it is near and fairly quiet (like a patch of turbulence) or distant and loud (like a faraway storm) is not clear yet.”
Eventually, solar-powered balloons could be used to explore other planets such as Venus. But here on Earth, Bowman plans to continue “surveying the data we’ve already collected to determine where the other sounds may be coming from.”
Follow Mike Snider on Twitter: @mikesnider.
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