The science of listening

When a whale sings to its mate in the South Pacific, or a volcano erupts in Iceland, chances are someone sitting in a room stacked with screens and computers at the UN offices in Vienna will hear it first. The Comprehensive Test-Ban-Treaty Organisation (CTBTO) was created to help enforce the 1996 treaty banning nuclear testing. To help do that it has created a global monitoring network.

"It's unprecedented - the largest verification regime in history and the most elaborate with over 300 monitoring facilities and the total investment in this is over $1 billion,” said Thomas Mützelburg, a public information officer at the CTBTO.

Sounds of science

As well as recording the fall-out from splitting atoms, the CTBTO is picking up sounds that are increasingly useful to scientists.

Mützelburg compares the scientific spin-offs from this nuclear snooping network to those delivered by the United States' Apollo space program in the 1960s – a program that gave us, among other things, solar cells and portable cordless vacuum cleaners. But the spin-offs from the CTBTO listening posts are in a different spectrum.

"Volcano scientists use our infrasound data to study the length and duration and strength of volcanic outbreaks..., maritime biologists use our data to track the paths of whales in the oceans," said Mützelburg.

It could be a storm

The CTBTO uses four different technologies to monitor the globe. There are seismic sensors for what's occurring underground while hydrophones listen for underwater events. For the atmosphere, there are ultra-sensitive nuclide detectors which are designed to pick up the tiniest amount of nuclear radiation. And then there are infrasound detectors.

"Yes infrasound is very interesting; for example, meteorological phenomena like thunderstorms, big storms also, can be measured with infrasound," said Gerhard Wotawa who heads the data and modeling division at Austria's Centre for Meteorology and Geodynamics.

Using infrasound data collected by the CTBTO's network of listening stations, Wotawa has been able to improve the tracking of storms across the Austrian countryside. It is "very very accurate," he told DW.

Long waves and long distances

Infrasound is beyond the range of the human ear. If we think of sound as waves in the atmosphere, most of what we hear comes from short waves. But infrasound is different.

"It's a huge wave - could be a hundred metres long to a few kilometers," CTBTO acoustic expert Pierrick Mialle said, "and it's much bigger than the human body, and that's basically why you can't hear it because it goes through your body without you feeling it or hearing it."

Those long-wave low-frequency sound waves are long distance travelers as well, allowing the CTBTO to snoop on what even the most secretive of regimes is up to. When North Korea triggered a nuclear explosion in 2013, Mialle and his colleagues actually "heard" the mountain move, even though the bomb exploded one kilometer underground.

He said the infrasound network was not designed for monitoring underground tests but since the explosion shook the ground this was sufficient for it to be recorded. "Because the ground acted, if you like, like a piston, like an acoustic source, and we were able to pick it up on two stations that were a few hundred kilometers away, to a thousand kilometers away in Japan."

Keeping aircraft out of volcanic ash

If, as Pierrick points out, infrasound can't be picked up by the human ear, how is it that he is posting the sound on the Internet? Well, this is infrasound speeded up and digitally processed to make it audible. The sound of the 2011 Tohoku earthquake which triggered the tsunami which wreaked havoc on Japan's coastline resembles a deep bass rumble, with a few squeaks and groans supplying higher notes.

As well as earthquakes the acoustic monitors are picking up the early rumblings inside volcanoes.

"For international aviation, it can be very important information, early warning of a volcano eruption, so that flights can be re-routed - encounters between aircraft and ash can be avoided. I mean this is very important," said scientist Wotawa.

The CTBTO is not being entirely altruistic in making its data available to scientists. The nuclear test ban treaty, which its network is designed to enforce, has not actually entered into force. And it won't until another half dozen countries, including the US and Iran, ratify it. That's not likely anytime soon. Stressing scientific spin-offs helps justify the costs of all this eavesdropping – costs which are met by the states that have signed the treaty.

"I think we are getting a better idea of how many meteors are now entering our atmosphere," said Jerry Carter, the senior performance and monitoring officer at the CTBTO.

That's because infrasound sensors are picking up the sound of meteors which would otherwise go undetected.

"Scientists and the people that are looking for the potential threats to earth from objects from space - it helps them make estimates of how many objects are out there that could be a threat to the Earth," said Carter.

And in a dramatic demonstration of their global monitoring network's ability to hear what humans can't, the CTBTO has posted infrasound of the asteroid which exploded over Russia last year on the Internet. Not the sound of the meteor's explosion, but of it tearing through the atmosphere before it became a fireball raining molten rock on the Ural Mountains.