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Sheets of plastic similar to that used for garbage bags, packing tape, some string, a little charcoal dust and a white shoebox-size box are more than odds and ends. These are the supplies Danny Bowman, a Sandia National Laboratories geophysicist, needs to build a solar-powered hot air balloon for detecting infrasound.
Infrasound is sound of very low frequencies, below 20 hertz, which is lower than humans can hear. African elephants produce infrasound for long-distance communication at around 15 hertz. For comparison, a bumblebee's buzz is typically 150 hertz and humans hear in the range of 20 to 20,000 hertz.
July - Fleet - Balloons - Height - Miles
Last July, a fleet of five solar-powered balloons reached a height of 13 to 15 miles, twice as high as commercial jets, and detected the infrasound from a test explosion. This experiment was funded by Sandia's Laboratory Directed Research and Development program. Bowman presented the results at the American Geophysical Union conference in December. The results will be published soon.
Infrasound is important because it's one of the verification technologies the U.S. and the international community use to monitor explosions, including those caused by nuclear tests. Traditionally, infrasound is detected by ground-based sensor arrays, which don't cover the open ocean and can be muddled by other noises, such as the wind. Bowman said air conditioners are also a common source of infrasound noise.
Stratosphere - Events - Interest - Security - Distances
"The stratosphere is much less noisy so you can detect events of interest to science and national security from greater distances," said Bowman. The stratosphere is the atmospheric layer from about 5 miles to 31 miles above the ground.
A solar-powered hot air balloon takes three hours for Bowman and fellow geophysicist Sarah Albert to make, and uses about $50 worth of materials, not including the reusable infrasound sensor or GPS tracker. The charcoal dust helps heat up the air inside the balloon,...
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