Strange sounds recorded high in Earth's atmosphere have scientists baffled

a view of Earth from a high-altitude balloon
A view of Earth from a high-altitude balloon. (Image credit: Matt Champlin/Getty Images)

Scientists have detected sounds high in Earth's atmosphere that can't be identified.

A solar-powered balloon mission launched by researchers from Sandia National Laboratories carried a microphone to a region of Earth's atmosphere found around 31 miles (50 km) above the planet called the stratosphere. This region is relatively calm and free of storms, turbulence and commercial air traffic, meaning microphones in this layer of the atmosphere can eavesdrop on the sounds of our planet, both natural and human-made.

However, the microphone in this particular study also heard strange sounds that repeat a few times per hour. Their source has yet to be identified. The sounds were recorded in the infrasound range, meaning they were at frequencies of 20 hertz (Hz) and lower, well below the range of the human ear. "There are mysterious infrasound signals that occur a few times per hour on some flights, but the source of these is completely unknown," Daniel Bowman of Sandia National Laboratories said in a statement..

Related: Earth's atmosphere: Facts about our planet's protective blanket

To gather acoustic data from the stratosphere, Bowman and the team used devices initially designed to monitor volcanoes called microbarometers that are capable of detecting detect low-frequency sounds.

Along with the expected natural and man-made sounds, the microbarometers detected the mysterious repeating infrasound signals.

The sensors were carried aloft by balloons that Bowman and fellow researchers built. Featuring diameters between 20 and 23 feet (6 and 7 meters), the balloons were built from common and inexpensive materials. Powered by sunlight, these deceptively simple devices were able to climb to altitudes of around 70,000 feet (13.3 miles) over Earth

"Our balloons are basically giant plastic bags with some charcoal dust on the inside to make them dark," Bowman said. "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."

Bowman explained that this passive solar power is enough to propel the balloons from the surface of the planet to the stratosphere. After launch, the balloons were tracked using GPS, something that the team had to do because balloons can often fly for hundreds of miles and may land in difficult-to-navigate regions of the planet.

And, as recent events have shown, scientific balloons can be mistaken for other objects, sometimes causing unintentional alarm.

Researchers with Sandia National Laboratories inflating a solar hot air balloon with an infrasound microbarometer payload. (Image credit: Darielle Dexheimer, Sandia National Laboratories)

In addition to helping further investigate these mysterious stratospheric sounds, solar-powered balloons like these could be used to investigate mysteries much further from Earth. 

Such vehicles are currently being tested to discover if they could be partnered with a Venus orbiter to observe seismic and volcanic activity through its thick atmosphere. Robotic balloons could drift through the upper atmosphere of "Earth's evil twin," high above its hellishly hot and high-pressure surface investigating its thick atmosphere and clouds of sulfuric acid.

The team's research containing the detection of these unidentified infrasound sources will be presented by Bowman on Thursday (May 11) at the 184th Meeting of the Acoustical Society of America being held in Chicago.

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Robert Lea
Contributing Writer

Robert Lea is a science journalist in the U.K. whose articles have been published in Physics World, New Scientist, Astronomy Magazine, All About Space, Newsweek and ZME Science. He also writes about science communication for Elsevier and the European Journal of Physics. Rob holds a bachelor of science degree in physics and astronomy from the U.K.’s Open University. Follow him on Twitter @sciencef1rst.

  • jbfly
    Bet it’s 6.66 hz and can be traced to a Russian station bouncing the signal off Starlink satellites.
  • blazeww420
    Maybe it's how sounds sent out on earth moves as waves and the different waves act like ocean waves interfering with eachother becoming stronger or weaker.
  • logix727
    Sounds like the pip or squeaky wheel old Soviet radio used to measure strat etc
  • Classical Motion
    I don't believe a human can here directly a one source, 1 to 20 Hz tone. But we can beat two tones and make one we can here. Such is the case when tuning a musical instrument. And tuning a CW radio signal. Two conventional audio generators can easily demonstrate this....and if you have tinnitus, only one generator is necessary.
  • diogeneskoolaid
    everyone knows it's the alien's communication grid...
  • robindahood
    Is it possible that the sound is a reaction from the electric magnetic field of the planet reacting with your balloon possibly causing a static discharge
  • curiosity_reads
    I agree with Robin hoods assessment in part, but would like to add or question if there is shielding on the mic or recording media? Is the period/frequency affected by time of day/location position of sun or any other weather phenomenon like storms high pressure low pressure air current channel etc etc?
    Has the sound been parsed by AI with timestamp meta data against other known time stamped databases in order to determine any potential sound origins? If this is really Sandia... you do have that ability yeah?
    But this is all BS.
    The real 2 issues that should be talked about are;
    a. wtf. Your 'researchers' wear tie dye and random overalls with semi hard baseball like hats in actual field work??? Or is this music video or burner party?
    b. Sound shmound. What is going on with the clouds in the photos? You aren't seriously going to tell me that the clouds are normal, not cgi, or don't have something a little odd going on ? I've seen same and photo them. they form oddly, progress unlike standard stratocirrus clouds and seem to be much much lower in cumulus territory then head in similar wedge from origin vectors but unscroll form in face, figure, letters. What is all that?
  • UbdU
    Better build that space wall before the aliens get into our planet. 😉
  • murgatroyd
    UbdU said:
    Better build that space wall before the aliens get into our planet. 😉
  • JustGuess
    Probably broadcasted from China....translated as "Hey dude, we gonna shoot down your balloon if it ever fly over to our country....."