US scientists have been sending home-made balloons with microphones 20 kilometres into the sky – and recorded sounds they can’t identify.
The stratosphere, from about 10 to about 50 km above the Earth’s surface, is a good place to hear sound: with few planes and little turbulence, it’s possible to pick up things like colliding ocean waves, thunder, wind turbines and explosions.
But this team has also detected a sound they haven’t yet been able to place.
“[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,” says Daniel Bowman, a researcher at Sandia National Laboratories, US.
Infrasound waves are too low-frequency for people to hear.
Bowman and colleagues have detected the sounds with solar-powered balloons , each six or seven metres in diameter.
“Our balloons are basically giant plastic bags with some charcoal dust on the inside to make them dark,” says Bowman.
“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 in the sky.”
The balloons are equipped with GPS trackers, so they can be easily collected, and “microbarometers”: devices that detect infrasound by spotting small changes in air pressure.
“Each balloon only needs about $50 worth of materials and can be built in a basketball court,” says Bowman.
Bowman is delivering preliminary results from the study at the 184th Meeting of the Acoustical Society of America.