Scientists Heard Deep Sounds in the Stratosphere. They Can't Say Where They Came From.

Aug 12, 2023

Scientists recently detected low-frequency sounds in our stratosphere with no known source.

Sounds that are undetectable anywhere else can often be detected in the stratosphere due to the atmospheric region's relatively calm nature.

Further observations will likely be needed to determine the sounds’ point of origin.

You’ve probably heard a fair amount in recent years about probing exoplanet atmospheres. The ticket to really beginning to understand other worlds in great detail, these atmospheres can tell you everything from what a planet is made of to whether or not it could support life.

But just because we’re probing other worlds’ atmospheres doesn't mean our own is without surprises. And recently, a group of researchers put that surprise on full display when they announced the detection of sounds in our stratosphere (the layer of atmosphere above the one we live and breathe in) that had no known source.

"[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," Daniel Bowman, a researcher at Sandia National Labs in New Mexico and leader of the team that discovered these signals, said in a press release.

Infrasound is very low-frequency sound—down toward or past the low end of human hearing capabilities. The team "spotted" these unidentified sounds using microbarometers attached to huge, solar-powered balloons.

Bowman said in the press release:

"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. 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."

Though they may seem low-tech, the balloons do what they need to do to properly listen—get up to the stratosphere. On the ground, and in lower levels of the atmosphere, many sounds can get lost in amongst other ambient noise or atmospheric disturbances.

But up in the stratosphere, there's a lot less going on, so you can hear a lot of sounds that would otherwise be blocked out. According to the press release, stratosphere-exclusive sounds can include "natural sounds from colliding ocean waves and thunder, human-created sounds like wind turbines or explosions, and even sounds with unknown origins."

Bowman and his team's work was presented at the 184th Meeting of the Acoustical Society of America, but that doesn't mean the "unknown origins" have become any more known. It will likely take further observations and analyses to eventually understand what was making these odd, deep sounds that resonate high up into our atmosphere. But until we do, it's just one more mystery keeping Earth interesting.

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