Mysterious sounds are recorded in Earth’s stratosphere using solar-powered hot air balloons.
Daniel Bowman of Sandia National Laboratories and his team create 6- to 7-meter-wide balloons to reach the stratosphere. Despite their size and capacity for data collection, the balloons are extremely simple.
The calmest layer of the Earth’s atmosphere is the stratosphere. Microphones in the stratosphere pick up a range of sounds that are unheard of anyplace else and are hardly ever disrupted by planes or turbulence. This includes sounds made by nature, such as thunder and the crashing of ocean waves, as well as sounds that result from human activity, such as explosions or wind turbines.
Daniel Bowman of Sandia National Laboratories will present his findings at the upcoming 184th Meeting of the Acoustical Society of America, running May 8-12 at the Chicago Marriott Downtown Magnificent Mile Hotel.
Microbarometers, which were first created to track volcanoes, are used by researchers to gather information and listen for low-frequency sound. After releasing the balloons, they use GPS to follow their paths, which is essential because the balloons can travel hundreds of kilometers and settle in inaccessible locations. However, because the balloons are affordable, simple to build, and easy to launch, they can release several balloons at once and gather more data.
Bowman said, “Our balloons are giant plastic bags with 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.”
Along with the expected human and environmental sounds, Bowman and his team detected something they could not identify.
Bowman said, “[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.”