A Mysterious Low-Frequency Sound Has Been Recorded High in the Stratosphere

A fleet of DIY solar-powered, high-altitude research balloons have recorded a mysterious low-frequency sound high in the upper atmosphere that repeats a number of times each hour.

The sound occurs in the infrasound range, meaning it’s below the threshold of human hearing, only occasionally spiking as high as 25 hertz (Hz) in frequency, and was recorded in the stratosphere at an altitude of 70,000 feet (21,300 meters) by special microphones called microbarometers that are typically used to monitor the low frequency sounds produced by volcanic activity.

Although these sounds repeated themselves a number of times per hour over numerous balloon flights, their origin remains a mystery: the stratosphere is a relatively calm layer of the atmosphere typically devoid of storms and turbulence, and is well above the maximum altitude attainable by airliners. Sounds from cyclical sources, such as ocean waves, wind turbines or thunderstorms were also ruled out.

The balloons that recorded the enigmatic sounds were built and launched by a research team led by Daniel Bowman, the Principal Scientist at Sandia National Laboratories. Rather than buying off-the-shelf weather balloons to carry their sensors aloft, Bowman and his crew built the solar-powered vehicles themselves out of everyday materials.

“Our balloons are basically giant plastic bags with some charcoal dust on the inside to make them dark,” Bowman explained. “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. Each balloon only needs about $50 worth of materials and can be built in a basketball court.”

The devices are tracked via onboard GPS devices, and their extremely low cost allow the team to launch numerous devices; perhaps with repeated recordings the source of these strange sounds will be one day revealed.