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.