Attendees gathered in Chicago, Ill., this week for the 184th Meeting of the Acoustical Society of America listened to a mysterious sound captured from the Earth’s stratosphere.
“There are mysterious infrasound signals that occur a few times per hour on some flights, but the source of these is completely unknown,” said Daniel Bowman of Sandia National Laboratories who presented findings from solar-powered hot air balloons Thursday at the Magnificent Mile Marriot.
Bowman and his team use the balloons to “eavesdrop on stratospheric sounds,” according to a press release about the research.
“The stratosphere is a relatively calm layer of Earth’s atmosphere,” it said. “Rarely disturbed by planes or turbulence, microphones in the stratosphere pick up a variety of sounds unheard anywhere else. This includes natural sounds from colliding ocean waves and thunder, human-created sounds like wind turbines or explosions, and even sounds with unknown origins. “
Located above the troposphere and below the mesosphere, the stratosphere extends from four to 12 miles above the Earth’s surface to around 31 miles. It holds 19% of the atmosphere’s gasses but has very little water vapor, according to the National Oceanic and Atmospheric Administration. The stratosphere is also where the ozone layer – which protects humans from UV radiation emitted by the sun – is located, per NASA.
Research balloons have already been in the news this year.
In February, the U.S. shot down what the government has identified as a “Chinese spy balloon” over the Carolina coast. According to Bowman’s team, their models are simple to make.
“Each balloon only needs about $50 worth of materials and can be built in a basketball court,” said Bowman.
With these materials, they created balloons that were around 20 feet across. He described the design as “basically giant plastic bags with some charcoal dust on the inside to make them dark,” made from painter’s plastic, shipping tape, and charcoal powder. This equipment is available at hardware and pyrotechnic supply stores.
“When the sun shines on the dark balloons, the air inside heats up and becomes buoyant,” the press release explained. Passive solar power brings the balloons from the Earth’s surface into the atmosphere.
Once they are up in the sky, the balloons collect data and detect low-frequency sound with microbarometers, devices originally designed to monitor volcanoes. Researchers track them with GPS as they “sail for hundreds of miles and land in hard-to-reach places.”
With this recent balloon expedition, Bowman said there were expected human and environmental sounds, as well as the “mysterious” sound his team wasn’t able to identify.
According to a CNN report, He said that mysterious sounds are likely “mundane, maybe a patch of turbulence, a distant severe storm, or some sort of human object like a freight train,” though it can be difficult to tell.
Last April, researchers from Sandia National Laboratory detected the “presence of an atmospheric acoustic duct, a channel of sound at the boundary between the troposphere and stratosphere,” for the first time. Infrasound signals within the acoustic duct were observed by hot air balloons.
Going forward, solar-powered balloons such as the ones Bowman’s team used could be used to explore the atmosphere of other planets, including the thick atmosphere around Venus, the press release said.
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