Low frequency sounds can travel vast distances across our planet, carrying information about the events that generated them as well as the medium through which they travel. These sounds are usually recorded on surface-based sensors. Recently, however, sensors have been lofted on high altitude balloons, where they have revealed a rich soundscape quite different than that of the Earth’s surface. Using inexpensive microbarometers lofted via passive solar hot air balloons, Sandia National Laboratories researcher Daniel Bowman and his colleagues detected background noise as well as individual infrasound events, some of whose origins remain enigmatic, in the lower stratosphere of our planet.
The stratosphere is a relatively calm layer of Earth’s atmosphere.
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.
To reach the stratosphere, Dr. Bowman and co-authors build balloons that span 6 to 7 m (20-23 feet) across. Despite their large size and data collection capability, the balloons are relatively simple.
“Our balloons are basically giant plastic bags with some charcoal dust on the inside to make them dark,” Dr. Bowman said.
“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 feet) in the sky.”
“Each balloon only needs about $50 worth of materials and can be built in a basketball court.”
The researchers collect data and detect low-frequency sounds with microbarometers, which were originally designed to monitor volcanoes.
After releasing the…
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