Infrasonic symphony: the greatest sounds never heard.

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, in large part because of the Comprehensive Test Ban Treaty, which was adopted by the United Nations in 1996. The monitoring section in this treaty calls for a global network of 60 infrasound-detecting stations to search for treaty violations (SN: 7/14/01, p. 25).

Each of the 24 monitoring stations established to date consists of an array of specialized infrasonic microphones that can detect the strength of a sound, its frequency, and the direction from which it's coming. The monitoring stations automatically send these data 20 times a second to the test ban treaty organization headquarters in Vienna, where computers pick out potentially interesting events. Scientists need to differentiate between infrasound from a meteor, a volcano, or a nuclear device.

"Ideally, we want to be able to say 'Here we have a signal, and we know it wasn't a nuclear test,'" says Michael A. Hedlin of the Scripps Institution of Oceanography in La Jolla, Calif., who heads the monitoring station in Pinon Flat, Calif.

This wealth of infrasound data isn't bound solely for Vienna. Scientists elsewhere are taking advantage of new infrasound-micro-phone arrays, both those within the nuclear-test monitoring network and at a handful of independent stations, to listen in on and study a variety of events in the atmosphere.

Last year, for example, 10 monitoring stations in the western United States and Canada recorded the explosion of the space shuttle Columbia. Some observers thought they saw lightning strike the shuttle or meteors explode nearby, but investigators discounted those reports because neither event showed up on infrasonic recordings, says Henry E. Bass of the University of Mississippi in University. Bass presented the shuttle data at the December 2003 American Geophysical Union meeting in San Francisco.

HEARING INAUDIBLE NOISES 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, dally, we sift and sort through these signals."

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

The swirling winds of hurricanes generate different infrasonic signals. Studying the rumblings of gathering storms could eventually lead to better prediction systems, suggests Hedlin. Researchers plan to use a future monitoring station in Cape Verde to study infrasound generated off the coast of western Africa, a known nursery for hurricanes destined for the East Coast of the United States.

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.

Olson, who presented infrasound data on these auroras at the December American Geophysical Union meeting, says he hopes lights in the sky.

A less-serene type of atmospheric storm poses a different opportunity for infrasound science. At NASA's Langley Research Center, Zuckerwar is investigating how to use infrasound data to warn airplane pilots of clear-air turbulence. These invisible patches of air are associated with jet streams and cause the bumpy plane rides that pilots try to avoid.

"Today, there's only one way to detect atmospheric turbulence, and that's when a pilot flies into it," Zuckerwar says. Currently, computer models can forecast clear-air turbulence, but there's no direct detection device.

Zuekerwar and his colleagues set up an army of four infrasonic microphones at the NASA facility. Bright orange casings protect the microphones from wind, which is one of the biggest problems for acoustic researchers. Much work went into finding the polyurethane material, which lets in infrasound but blocks the wind.

The microphones constantly record infrasonic wavelengths that pass over the arrays, and once a week the researchers download the collected data. Then, one member of the team looks for patterns in the infrasound record that correspond to pilot reports of turbulence or predicted turbulent areas within a 300-kilometer range of the microphones. The researchers haven't reported any infrasound signatures yet.

Although Zuckerwar emphasizes that the research is in an early phase, his goal is to establish infrasonic monitoring stations, probably one every 200 km or so turbulence-prone areas. Flight controllers would pick out characteristic turbulence readings and quickly notify pilots of the hazard.

EAR TO THE GROUND 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 volcanos lava lake is generated by the rupture of 10-meter-wide, gas-filled 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."

Studying the patterns of infrasound that precede eruptions might also have predictive value. While placing infrasound sensors on the Sakurajima volcano in Japan, Garces witnessed an unexpected series of increasingly frequent and powerful explosions. By the end of the day, Sakurajima erupted.

"We collected some really good data," said Garces, "and demonstrated there is a relationship between the increasing amplitude of a wave and how often these events occur" leading up to an eruption.

If researchers monitoring a volcano learn when it's about to erupt, they could warn nearby residents and pilots scheduled to fly in the vicinity. The eruption endangers people on the ground and spews volcanic ash that could bring down a jet (SN: 9/13/03, p.168).

Tests are under way to use infrasound data as a warning signal for avalanches as well. When snow rushes down mountains, it pushes air before it and creates infrasound at frequencies below 8 Hz.

Ernie Scott of IML Air Science in Sheridan, Wyo., set up infrasound-detecting microphones in Teton Pass, Wyo., an area prone to avalanches. State officials there frequently set off explosives to create minor cascades that defuse snow buildup. Scott recorded these triggered avalanches and used the patterns to design a prototype detection system for the Wyoming Department of Transportation. This winter, the agency will use the system to monitor activity over a one-square-mile area of Teton Pass, where avalanches are a frequent wintertime hazard to skiers and drivers. The data will be converted to radio signals transmitted to highway personnel 15 miles away.

"Essentially, they can sit there and listen for an avalanche to occur," says Scott. "If it hits a highway, they can send the road crews out."

An infrasound monitoring system could also immediately alert rescue units of avalanches that may have trapped skiers.

The goal of IML Air Science is to market an avalanche detection device to snowy states and countries.

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. Says Bass: "Everybody out here is excited about something different."

Animal acoustics Creatures communicate with rumblings that people sometimes feel

Just because people can't hear infrasound doesn't mean other animals can't. In the early 1980s, Katy Payne of Cornell University and her colleagues found that elephants' rumbling vocalizations contain pressure waves at frequencies as low as 14 hertz that can travel up to 10 kilometers across forests and savannas. The researchers suggested that elephants communicate over long distances via infrasound.

It's still unclear how elephants create or detect infrasound, but Caitlin E. O'Connell-Rodwell of Stanford University is examining the idea that an infrasound portion of elephant calls is transmitted through the ground to be picked up by distant elephants' feet (SN: 3/24/01, p. 190).

"We noticed a pattern of behavior before the arrival of an [infrasonic] event," says O'Connell-Rodwell. The elephants "shift the weight on their feet and lean forward, as if they were paying attention to the ground with their feet," she says. O'Connell-Rodwell and her colleagues found that elephant rumblings cause waves that propagate through the ground as well as through the air. The seismic waves, which can travel 4 kilometers to 16 km, may extend the reach of elephant communication.

Low-frequency communications have also been linked to whales and rhinos--other large animals that produce powerful sounds. Some scientists have hypothesized that dinosaurs could generate and pick up infrasound. Now, research suggests that some birds create these ultrabass notes as well.

In the October 2003 Auk, Andrew L. Mack of the Wildlife Conservation Society in Papua New Guinea and Josh Jones, now of Scripps Institution of Oceanography in La Jolla, Calif., report that the large, flightless cassowary emits borderline-infrasonic calls as low as 23 Hz.

Cassowaries live solitary lives in the rain forests of Papua New Guinea and Australia, and Mack proposes that their deep sounds reach neighbors and potential mates through thick vegetation. He's currently investigating how cassowaries detect infrasound, paying particular attention to the large, pointy casque atop each bird's head.

While people can't hear infrasound, they apparently detect it in other ways. Mack describes a "strange sort of sensation" from standing near a calling cassowary, and O'Connell-Rodwell says that when elephants rumble, "it's such a powerful, low-frequency sound, you really feel it resonating in your chest."

A recent acoustic experiment in England tested people's responses to unrecognized infrasound. A team of acousticians, psychologists, and musicians rigged a large pipe to produce 17-Hz waves, which they played during selected contemporary music pieces being performed in a concert hall. The 750 concertgoers later answered questions about emotions or strange feelings that they may have experienced during the pieces. The scientists presented their findings in September 2003 at the British Association for the Advancement of Science's Festival of Science at the University of Salford in England.

During pieces accompanied by infrasound, "the effect was to intensify the current emotional state" of the listeners, says acoustician Richard Lord of the National Physical Laboratory in Teddington, England. The investigators suggest this could explain why some pipe organ music can elicit powerful emotions in people.--K.R.