High-Altitude Balloons Discover a ‘Mystery’ Sound in the Sky

Scientists have captured the sounds of the skies at an altitude of 13 miles with microphones attached to solar-powered balloons. The aerial recordings include the rumblings of thunder, wind turbines, and ocean waves, as well as a mysterious low-frequency “infrasound” vibration with no clear origin.

By eavesdropping on this calm atmospheric layer, called the stratosphere, researchers can hear the disturbances of human activities and natural processes far below on Earth’s surface. This approach can reveal insights about events on the surface, including industrial explosions or volcanic eruptions, that can be missed from the ground.

The newest results of the balloon flights will be presented on Thursday by Daniel Bowman, a principal scientist at Sandia National Laboratory, at the 184th meeting of the Acoustical Society of America in Chicago. Bowman and his colleagues have been recording sky sounds for years by launching a series of low-cost balloons over the American Southwest.

“Our balloons are basically giant plastic bags with some charcoal dust on the inside to make them dark,” Bowman said in a statement. “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. This passive solar power is enough to bring the balloons from the surface to over 20 km (66,000 ft) in the sky,” he added. “Each balloon only needs about $50 worth of materials and can be built in a basketball court.”

Bowman and his colleagues focus on infrasound waves, which are low-frequency acoustic vibrations that are mostly beyond our human hearing range. Infrasound waves can travel for tens of miles across Earth’s surface, and they also reverberate upward into the skies, making them a useful way to tap into disruptive events over long ranges.

In addition to listening in on identifiable sounds, the balloon flights have also found at least one unexplained rumble in the stratosphere.

“There are mysterious infrasound signals that occur a few times per hour on some flights, but the source of these is completely unknown,” Bowman said in the statement.

Future balloon flights may pin down the origin of these weird signals, and expand our understanding of Earth and perhaps, other planets.

“Our work has implications for monitoring remote regions of the earth for explosions, volcanic eruptions, and other phenomena,” Bowman and his colleagues said in a 2021 study. “It also supports the prospect of balloon-based infrasound seismology on Venus.”