Untrue as it is, there is something wonderful about the notion that you can hear the ocean when you hold a conch to your ear—a synecdoche of shell for sea, a beguiling fantasy that if you listen hard enough you can hear the womb-sound of the world. The systems used to study infrasound are less poetic; the ones I’ve seen look like little tarmac car parks with sawn-off drainpipes sticking up out of them. But they make the seashell’s promise real. They let you hear far-off oceans.
Infrasound is, like sound (and, for that matter, ultrasound), a matter of vibration. The difference is that infrasound vibrations are slower, which makes the pitch of the sound too deep for the human ear to make out—and the distance the sound can travel much longer. Nine-tenths of the energy in a sound wave at a frequency of 1,000HZ (the C above middle C, more or less) is absorbed by the air within seven kilometres. Absorbing that much of the energy in an infrasound signal at 1HZ takes 3,000 kilometres.
Things that make a vast amount of audible noise often make inaudible infrasound, too. The 1883 eruption of Krakatoa, in Indonesia, is reckoned to have been the loudest sound in recorded history, audible a few thousand kilometres away, physically deafening within 20 kilometres or so. Its infrasound, the first measured as such, went round and round the world, picked up by state-of-the-art barometers as tiny fluctuations in air pressure. Other loud things—thunderstorms, quarrying, battles and the like—subsequently turned out to produce infrasound as well.
In the 1930s Hugo Benioff, who, with his Caltech colleague Charles Richter, was revolutionising the study of earthquakes, found that there was infrasound that had no audible counterpart. His microphones picked up regular little atmospheric trills he called “microbaroms”. These are now thought to come about when ocean waves headed in different directions interfere with each other, causing a patch of the surface to rise and fall like a loudspeaker. The northern lights turn out to be another inaudible source of ultrasound. So are some winds, like the Chinooks of Canada, and the ghostly “sprites” of ionised plasma that rise up into the high atmosphere above thunderstorms, too faint to see with the naked eye. Speed their infrasound up to audible frequencies and you hear rather sweet oops-y noises, very different from the thunder of the storms below.
After the second world war, and before the age of satellites, infrasound took on a new significance as a way of eavesdropping on nuclear tests. The wreckage which came to earth at Roswell, New Mexico, in 1947—fons et origo of endless UFO speculation—was part of a stratospheric balloon system with which the United States Air Force was listening out for the first Soviet bomb. By the early 1950s labs in Washington, DC, could record American nuclear tests in the Pacific and bike the tapes over to President Eisenhower’s White House.
With the advent of satellite monitoring and the 1963 ban on atmospheric nuclear tests, infrasound lost much of its national-security appeal, and thus its funding. But as Michael Hedlin and his colleagues explain in a recent overview of the field, the creation in the past few years of a comprehensive global monitoring system to police the more general Comprehensive Test Ban Treaty has given it a new lease of life. As well as seismometers and systems for monitoring sound underwater (which can also travel extraordinary distances), this international monitoring system will have about 60 infrasound stations, spread on average 2,200km apart all around the world.
Just as satellite technologies once used to spy on enemies are now used to probe the environment, so it may be with these infrasound stations. Infrasound does not spread evenly; it depends on the direction and speed of winds in the stratosphere, 10 to 50km up, and conditions in the thermosphere, higher still. Seasonal changes in the upper atmosphere mean that in the northern hemisphere it is easier to hear infrasound coming from the east in summer, from the west in winter. By picking up on subtler ways in which infrasound is distorted and redirected by the upper atmosphere, it should be possible to use microbaroms and other relatively common sources to put together profiles of the winds at very high altitudes, which are currently hard to measure both from the surface and from space.
Not paying enough attention to the stratosphere has been a problem for many models of the climate. It’s possible that a better understanding of its highest winds could help put that right. And there might also be unconsidered discoveries to make. Scientists did a great job turning the satellites and seismometers called for by national security into new ways of seeing the Earth’s surface and its depths. Infrasound could do the same for the structure of the atmosphere’s invisible, but not inaudible, heights.
The sound of the world as a whole may never rival the sight of the Earth from space as an inspiration; that people attend to its unheard sound, though, should add an extra susurrus of wonder to every seeming silence.