There are sounds in the stratosphere that researchers cannot explain. © IMAGO/Design Pics
With the help of a simple solar-powered balloon, a research team takes pictures in the stratosphere. Not all sounds can explain them.
Chicago – Depending on where you are on Earth, it's extremely noisy. Noise from cars, trains, construction sites or noises such as birds chirping or children playing can be heard. However, humans can only find out what it sounds like far above Earth indirectly – with the help of microphones that are transported into the stratosphere by special balloons, for example. This is exactly what a research team led by Daniel Bowman (Sandia National Laboratories) has done, the research results were presented at the 184th meeting of the Acoustical Society of America in Chicago.
Bowman's team is launching solar-powered balloons into the stratosphere, at an altitude of about 21 kilometers. This region is relatively quiet – there are hardly any turbulences, storms and flight operations there. There, a microphone can eavesdrop on the sounds of the earth undisturbed. The microphones pick up sounds such as waves or thunder, but also man-made sounds such as explosions.
Mysterious sounds discovered in the stratosphere
However, Bowman's team has not yet been able to identify all recorded sounds. "There are mysterious infrasound signals in the stratosphere that can be heard a few times per hour during some flights," explains the researcher in a statement. "The source of these noises is completely unknown." The unidentified sounds were recorded in the infrasound range, at a frequency of 20 hertz and lower, far below the range that the human ear can perceive.
The microphones used are so-called microbarometers, which were actually manufactured for monitoring volcanoes. They are capable of detecting noise at very low frequencies. The balloons themselves were made by the team themselves using cheap materials: "Our balloons are basically huge plastic bags, with some coal dust on the inside to make them dark," says Bowman. "We build them from painter's plastic from the hardware store, parcel tape and charcoal powder from pyrotechnic stores. When the sun shines on the dark balloons, the air inside heats up and the balloon floats."
Inexplicable noise in the stratosphere repeats itself
In the future, solar-powered balloons could also investigate mysteries beyond Earth – for example, tests are being carried out to see whether they could be used on Venus. Robotic balloons could fly through the upper atmosphere of Earth's neighboring planet and collect data for research. (tab)