Passage IV

NATURAL SCIENCE: This passage is adapted from the article "Infrasonic Symphony" by Kate Ramsayer (©2004 by Science Service).

"Let me start off with a riddle," says NASA scientist Allan J. Zuckerwar. In his office in Hampton, Va., he rattles off items as dissimilar as rhinoceroses, supersonic aircraft, and hurricanes. "Now, what do they have in common?" The answer, Zuckerwar explains, is that each one generates silent infrasound—long sound waves at a frequency below 20 hertz. People can't hear anything below that frequency, probably for good reason. Otherwise, they'd be bombarded by the constant din of wind, the intermittent groaning of Earth, and the occasional distant explosion. But scientists are eavesdropping on volcanoes, avalanches, earthquakes, and meteorites to discern these phenomena's infrasound signatures and see what new information infrasound might reveal.

Just as seismic waves travel through Earth, infrasonic waves travel through the air. And the lower the frequency of the waves, the farther they can travel without losing strength. Scientists first detected infrasound in 1883, when the eruption of the Krakatoa volcano in Indonesia sent inaudible sound waves careening around the world, affecting barometric readings.

Infrasonic research gained significant attention and funding in the 1950s, when the United States and the Soviet Union used infrasound to detect each other's atmospheric nuclear testing. Interest declined when aboveground bomb testing was banned in 1963 as part of the Limited Test Ban Treaty.

But lately, scientists have turned back to infrasound.

Infrasound interpretation is a young science. Acousticians and geophysicists are still learning what phenomena generate infrasound signatures and how to match signatures with phenomena.

For example, John V. Olson of the University of Alaska in Fairbanks recalls one morning last April when a colleague rushed into his office and asked whether he had heard an explosion the night before. The two scientists found a large pulse on the infrasound record from the nuclear-test monitoring station that the university operates and traced it to a nearby firing range. The next day, the local paper reported that a citizen had found a bundle of dynamite, which police exploded at the range.

"So, we take [the signal] out of the 'little green men' file and say, 'This is what dynamite looks like from 5 miles away,'" says Olson. "Slowly, daily, we sift and sort through these signals."

Ocean storms and waves are two of the big generators of infrasound, says Milton A. Garcés of the University of Hawaii at Manoa. The routine up-and-down movements of the waves act as a giant loudspeaker, pushing the air at infrasonic frequencies.

Low-frequency sounds are also generated by one of the most colorful displays in the sky, the northern lights, which are caused by charged particles in the air. This electricity heats atmospheric gases, and the warmed gas molecules spread out and increase air pressure.

"It pushes the neutral air forward, almost like the bow wave off a ship," says Olson. This air movement creates an infrasonic signal. The readings are visible during the beginnings of these magnetic storms, as the bright, greenish lights sweep across the sky like a fluttering curtain.

While specialized microphones can pick up infrasonic signals generated high in the atmosphere, they detect more earthly rumbles, as well. For instance, Jeffrey B. Johnson of the University of Hawaii at Manoa has placed microphones within a kilometer of a vent of the active Erebus volcano in Antarctica. The sensors have recorded low-frequency signals so powerful that, were they audible, they'd have a volume in excess of 130 decibels—"somewhere between a jet airplane and the threshold of pain," says Johnson. Erebus does produce some audible sound, but it's not very loud, he says.

The infrasound radiating from the volcano‘s lava lake is generated by the rupture of 10-meter-wide, gasfilled bubbles, which pushes huge infrasound waves into the atmosphere. Johnson can use infrasound readings to estimate the size of the lava bubbles within Erebus and the amount of gas they contain.

"Infrasound is a powerful tool to understand more about explosions and eruption sources," says Johnson. "It allows us to directly quantify what's going on at a volcanic vent."

Whether infrasound is used for commercial purposes, to learn more about natural phenomena, or simply to listen for something that nobody wants to hear, it is entering what those in the field call a renaissance. Geophysicists and acousticians are sorting through, categorizing, and studying a wide range of inaudible noise.