The source of the mysterious sounds is unknown. There is no wind and usually no visible movement of air masses. But the microphones recorded strange noises that were repeated several times an hour.

details of the study

A solar-powered balloon launched by researchers at Sandia National Laboratories carried the microphone into the stratosphere 50 kilometers above the planet. The stratosphere is a relatively calm layer of Earth’s atmosphere. Here the microphones pick up many sounds that cannot be heard anywhere else, as they are not disturbed by airplanes or turbulence. In the stratosphere, you can hear natural sounds of ocean waves and thunder, as well as anthropogenic sounds such as wind turbines or explosions.

The team used devices designed to observe volcanoes – microbarometers – to collect acoustic data from the stratosphere. They can detect low frequency sounds.

Along with the expected natural and man-made sounds The microbarometer recorded mysterious repetitive infrasound signals at frequencies of 20 hertz and below.is significantly below the sensitivity range of the human ear. Sources not identified.

Our balloons are basically giant plastic bags with some charcoal dust to darken them. We build them using painter’s plastic from the hardware store, shipping belt, and coal dust from pyrotechnics stores. When the sun illuminates the dark spheres, the air inside heats up and becomes buoyant,
— Daniel Bowman of Sandia National Laboratories explained.

A balloon in various stages of operation / Photograph by Darielle Dexheimer, Sandia National Laboratories

Passive solar energy is enough to lift balloons from the planet’s surface into the stratosphere. After launch, the projectiles were tracked using GPS. Such shells can be launched in large numbers, as the devices are easy and inexpensive to make.

Scientists hope the next batch of balls will help them learn more about mysterious low-frequency stratospheric sounds. Until now, scientists don’t even have a guess about their nature.