Scientists from Sandia National Laboratories discovered mysterious low-frequency signals by sending a balloon with a microbarometer capable of picking up low-frequency sounds into the stratosphere. The researchers wanted to record natural and man-made sounds, but the devices unexpectedly picked up repetitive infrasound signals at frequencies of 20 Hz and below at an altitude of about 50 km.
The stratosphere is the second layer of the Earth’s atmosphere, relatively calm, but through it you can hear faint sounds coming from the troposphere where we live. Most often these are the sounds of explosions, falling meteorites, airplanes, cities and thunderstorms. However, when scientists from the Sandia National Laboratory (USA) filled balloons to explore the stratosphere, they recorded strange low-frequency sounds that they had not heard before.
Mysterious sound in the stratosphere
Sandia National Laboratory scientists have been launching balloons into the stratosphere and recording infrasound since 2016. Since then, more than 50 ballons have been launched. Initially, the researchers studied the sounds of volcanic eruptions, but then began to pay attention to other noises. In particular, during such flights, they discovered a strange repetitive sound, similar to a hum, that occurs several times an hour.
Its source cannot be traced, unlike all other sounds that are recorded by devices. There is no wind and no noticeable movement of air masses at all. However, the microphones picked up strange sounds repeating several times per hour. The researchers reported this at the 184th meeting of the American Acoustical Society.
Researchers have some speculation about the origin of this sound. According to one version, it may be an unknown form of atmospheric turbulence. In addition, the noise may be an echo distorted beyond recognition. However, it is not clear why the sounds were periodically repeated. Therefore, it is too early to draw conclusions about their origin.
A solar-powered balloon launched by researchers at Sandia National Laboratories carried the microphone into the stratosphere, 50 km above the planet. The stratosphere is a relatively calm layer of the Earth’s atmosphere. Microphones pick up a lot of sounds here that are not heard anywhere else, as they are not interfered with by aircraft or turbulence. In the stratosphere, you can hear the natural sounds of crashing ocean waves and thunder, as well as man-made sounds such as wind turbines or explosions.
To collect acoustic data from the stratosphere, the team used devices originally designed for observing volcanoes – microbarometers. They are able to detect low frequency sounds. Along with the expected natural and man-made sounds, the microbarometer registered mysterious repetitive infrasound signals at frequencies of 20 Hz and below, well below the sensitivity range of the human ear. Their source has not been identified.
“Our balloons are basically giant plastic bags with a little bit of charcoal to make them dark. We construct them using painter’s plastic from a hardware store, shipping tape, and charcoal powder from fireworks stores. When the sun illuminates the dark orbs, the air inside heats up and becomes buoyant,” explained Daniel Bowman of Sandia National Laboratories.
Passive solar energy is enough to lift balloons from the surface of the planet into the stratosphere. After launch, the balls were tracked using GPS. Since the devices are easy and cheap to build, large numbers of such balloons can be launched. Moreover, according to the researchers, such balls can also be used to observe other planets. For example, to study the seismic and volcanic activity of Venus, which has a dense atmosphere.
Scientists hope that the next batch of balloons will help them gain more information about the mysterious low-frequency stratospheric sounds. So far, experts have no assumptions about their nature.