Sometimes, the most important sounds are those that cannot be heard.
Take infrasound—acoustic waves below the range of human hearing. Although nuclear weapons blasts, midair meteor explosions, volcanic paroxysms, and angry thunderstorms make plenty of noise people can hear up close, the infrasound these phenomena emanate can also circumnavigate the globe. Even if a scientist is half the world away, their infrasound detector may be able to pick it up.
Despite its promise as a remote sensing technique, you can’t register these sources of infrasound everywhere. The world’s oceans are not only cacophonous, but the absence of land—particularly within the Southern Hemisphere—has made placing detectors a seemingly insurmountable challenge. But for Olivier den Ouden, an acoustics researcher at the Royal Netherlands Meteorological Institute, the solution to this conundrum was obvious: put infrasound sensors into tiny backpacks and get albatrosses to wear them.
Turning the Southern Ocean’s largest seabirds into cyberpunk spies “was a shot in the dark,” says den Ouden. But as his team reported this August in the journal Geophysical Research Letters, it actually worked. As those feathered friends flitted above the frigid waters midway between southern Africa and Antarctica, the instruments in their backpacks recorded various sources of infrasound, suggesting that it is possible to listen for all sorts of distant booms without requiring any land to site detectors.
When Daniel Bowman, a geophysicist at Sandia National Laboratories in Albuquerque, New Mexico, first read the paper, he recalls saying, “You’ve got to be kidding me.” But by the time he had completed his peer review, he was convinced of the team’s claims. “I couldn’t believe it,” he says.
To be fair, infrasound has often revealed the secrets of far-flung things. When volcanoes erupt, they act like musical instruments: The expulsion of molten rock and the tumultuous plumes of ash and gas propel the atmosphere out of the way, creating waves that volcanologists can use to detect the onset and evolution of distant eruptions.
“We have a lot of erupting volcanoes in Alaska,” says Alex Iezzi, a geophysicist at UC Santa Barbara who was not involved with the study. “And you can’t put instruments on every single one of those volcanoes and be able to maintain them all the time.” But detectors hundreds of miles away can hear this eruptive infrasound just fine, and there is no risk they will be annihilated by volcanic fury.
Other large explosions—like the one that scarred the city of Beirut last year—also generate infrasound. Any explosion above ground transmits most of its energy to the atmosphere. That means the infrasound of a chemical blast can be used to quickly determine its energy release in terms of tons of TNT, says Oliver Lamb, a geophysicist at the University of North Carolina at Chapel Hill who was not involved with the study.
In a decidedly more tranquil manner, a variety of animals—elephants, tigers, and peacocks, for example—are known to communicate using infrasound. By listening to their vocalizations, scientists hope to be able to both better understand these beasties while tracking them from afar—a technique that may reduce the need to approach circumspect critters and place physical trackers on them.
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Recording infrasound on land isn’t particularly tricky; you can place sensors pretty much anywhere. Not so in the oceans of the Southern Hemisphere: Sensors can only be placed on mostly small, lonely islets, so the coverage is poor.
And, den Ouden says, out in the open ocean the “huge chaos of waves” makes a lot of undesired noise. Some of this irritating infrasound comes about as the sloshing sea surface waves interact. “The ocean starts to go up and down with a rhythm,” says den Ouden. The sea acts like a gigantic speaker, blasting energy into the atmosphere that travels upwards and across the water, toward the land, like an invisible tidal wave. Other oceanic infrasound is less problematic but more mysterious: The motion of the sea triggers atmospheric vibrations that radiate straight upward. But these waves have proven so difficult to detect that their existence has long been an open question.
This collection of infrasound waves, which are technically known as microbaroms, have been referred to as the “voice of the sea.” Most researchers want to drown it out. “We try to get rid of the microbarom signal, because we’re interested in explosions,” Iezzi says.
Ideally, infrasound detectors at sea would not only be able to fill in a vast coverage gap, but also document the microbaroms well enough that, with the help of filtering software, they could be effectively canceled out. But where would you put these detectors? Boats wouldn’t work. “The problem with them is that they’re moving up and down all the time,” says Lamb—and that would mess with the recording. Balloons have been used to record infrasound on land, but their flight paths over the sea would be too unpredictable to be of any use. (They would, however, be useful for recording lightning strikes, quakes, and volcanic eruptions on Venus, because the surface of Earth’s evil twin is so hot that any instruments placed on the ground there would quickly melt. Or, at the very least, overheat.)
The open ocean is “an extremely challenging place to record sound,” says Bowman, “so challenging, in fact, that if you’d asked me prior to looking at this paper, I’d have said it’s basically impossible.”
As it happens, Samantha Patrick, a seabird ecologist at the University of Liverpool, was curious about the ability of seabirds to navigate using infrasound. After conversing with den Ouden and his weather- and geophysics-focused colleagues, they developed an outré idea: Why not attach microbarom detectors to birds? And not just any birds: wandering albatrosses. Their wingspans, which can be 11 feet long, are lengthier than any human is tall. This allows them to spend considerable time simply floating on air currents above open waters, something that conserves energy as they embark on foraging trips. Not only do they fly across vast swaths of isolated ocean, but they don’t dive into the water, so any sensors attached to them wouldn’t get especially wet.
In short order, the researchers built minuscule infrasound sensors and fitted them into pouches—packages no more hefty than a TV remote. As fun as it may be to visualize these bags being lugged about the way a school kid carries a backpack, that would have been needlessly complicated. Instead, the pouches were simply stuck to the backs of the avian assistants with some duct tape.Last year, the team headed to the Crozet Islands, little blips of land in the French sub-Antarctic on which wandering albatrosses like to nest. But how, pray tell, do you get albatrosses to cooperate? With a very special sort of hug, apparently—one that prevents any potentially injurious flapping and pecking. “They don’t really have predators—certainly no natural predators,” says Patrick, who assisted the team with their research. “So you literally just walk up to it, and then you put your hand on its bill, and then you have to hug it, because it’s so big. You give it a hug and lift it off the nest, and then one person holds it, and then the other person duct-tapes the logger to their back.”
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“And that’s it,” Patrick concludes with a shrug.
Throughout 2020, as the albatrosses quested for food, they flew 25 battery-powered infrasound packs over the Southern Ocean, with each individual round trip taking about 15 days. When the birds returned home, the team carefully removed their pouches and downloaded the data. Altogether, they collected 115 hours of recordings as the albatrosses wandered a total of 26,200 miles.
Now the team had an infrasonic soundscape of a huge section of the Southern Ocean, one that featured not only the microbaroms that propagated over immense distances but also those evanescent ones whose existence was, up until this point, uncertain. The immediate benefit is that the microbarom signal, now better documented than ever before, can be more precisely filtered out of infrasound recordings so that other sources, be they eruptions or explosions, can be more clearly identified.
So in the future, when it comes to detecting infrasound, will cyberpunk albatrosses become the new industry standard? “This is a good proof of concept,” says Lamb. But using wild animals to conduct scientific labor in the long-term is an ethical minefield that few would dare to cross—such prolonged and intensive contact with humans may risk triggering changes in the animals’ long-term behavior. “It would be great if we can equip all the birds and sea turtles in the Southern Hemisphere with these loggers, but that’s not gonna happen,” den Ouden says.
The next step, then, probably involves yet more clever technical wizardry: creating something that flies long-distance like an albatross, but doesn’t entail the same ethical issues. “I wouldn’t be surprised if there’s an engineer somewhere trying to work out a way to make a drone fly like an albatross where they just glide,” Lamb says.
Still, for infrasound aficionados, the most important aspect of this study is its creativity in trying to solve what appeared to be an unsolvable problem. “No one bothered to ask the question, because we all thought we knew the answer,” says Bowman. “They’ve opened a window to what might be possible.”
A few years ago, Iezzi met den Ouden at a conference, and he outlined his ambitious plans to record oceanic infrasound. “He’s like, ‘I’m building these little backpacks for these birds, and I’m just going to put it on them,’” Iezzi recalls. She thought it sounded crazy. “But my gosh, he actually did it and he got useful data out of it,” she says.
“It’s a fun story,” says den Ouden. “But it’s only fun because eventually it worked out.”
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