Denver - In rumbling tones far lower than the sounds that human ears can hear, a symphony of mysterious noises constantly assails the entire globe, and scientists are learning to translate and even to exploit the inaudible signals.

Avalanches rumbling down mountainsides, the seething magma inside volcanoes as eruptions near, the violent twisting air of tornadoes and even the crash of ocean waves -- all send out ultra-long-wave, low-frequency vibrations known as infrasound.

Using sensitive instruments, scientists are studying whether detection of infrasound can provide advance warning of these natural disasters. They also are using infrasound to detect the nuclear weapon tests of enemy nations.

Infrasound is the opposite of the extreme high-frequency whistles that dogs can hear but humans can't, or the high-pitched ultrasound echolocation that bats use home in on their prey or that modern physicians use to create images of a fetus in the womb.

All sound waves are actually pulses of air pressure -- rapid pulses in the case of high-frequency sounds, and very slow-moving pressure waves with extremely long wavelengths in the infrasound ranges.

The frequency of sound waves are measured in hertz, after the 19th century German physicist Heinrch R. Hertz, who first measured them. The human ear can normally discern sounds ranging from a low of 20 Hz to a high of 20,000 Hz. Infrasound signals start below 20 Hz, but can be detected at frequencies as low as a hundredth, or even a thousandth of a hertz.

While ultrasound signals have a relatively short range and last only briefly, the long-lasting pressure waves of infrasound can be picked up by sensitive barometers at extraordinarily long distances.

When the giant Indonesian volcano Krakatoa erupted in 1883, for example, its infrasound waves were detected traveling more than three times around the globe. And two years ago, waves churned up by a tropical storm off Mexico generated infrasound signals that were detected by stations in Boulder, Colo.

At the annual meeting of the American Association for the Advancement of Science last week, Alfred J. Bedard Jr. described how he and his colleagues at the federal Environmental Technology Laboratory in Boulder are trying to use a network of infrasound detectors to warn of oncoming tornadoes.

The funnel-shaped columns twist powerfully and emit strong infrasound signals as they race above the countryside, threatening small towns and farms in the Midwest. Infrasound can signal their approach many minutes before they reach the ground, Bedard said.

Bedard's laboratory is also contracting with commercial firms to apply the technology at ski areas in the Rockies and the Sierra Nevada where inaudible rumblings from early avalanche movements can warn back-country skiers to stay away from the early buildup of downslope snow movements.

Most recently, Bedard and his colleagues reviewed infrasound data to see if they could detect anomalies in the infrasonic boom of the doomed space shuttle Columbia during its re-entry into Earth's atmosphere over California.

The most sensitive infrasound detection network is used to listen for nuclear weapon tests around the world. As the threat of such testing grows, more stations are being built, said Douglas Christie of the United Nation's Comprehensive Nuclear Test Ban Treaty Organization, known as the CTBTO.

Nearly 100 nations have ratified the treaty, and another 59 have signed it. Although the United States signed it during the Clinton administration, the Senate has refused to ratify it.

More than 60 test ban monitoring stations operate on every continent and even on small islands around the world, Christie said. Another half-dozen are under construction. The stations include a variety of instruments, including seismographs that can pick up signals from nuclear weapon blasts underground.

The U.N. nuclear test ban agency has tested the sensitivity of the infrasound detectors by recording the low-frequency rumbles of exploding meteors, known as bolides, as they enter the Earth's atmosphere.

Bolides explode with enormous energy. Douglas O. ReVelle, a meteorologist at Los Alamos National Laboratory, estimates that every year at least one bolide explodes over the Earth's surface with the force of 15 kilotons, or 15, 000 tons of TNT. (The atomic bomb dropped on Hiroshima in World War II was equivalent to about 12,500 tons of TNT).

Infrasound signals from at least five such bolides have been detected and located nearly 5,000 miles away, Revelle said. Los Alamos scientists have now installed low-frequency sound detectors to watch for bolides exploding over Los Angeles, Utah, Wyoming and at the Nevada nuclear weapon test site near Las Vegas.

Milton Garces, a physicist at the University of Hawaii, is studying how infrasound detectors might be useful to warn when volcanoes are ready to erupt -- because the lava moving up their throats emits powerful bursts of infrasound.

Those signals could give as much as a 30-minute warning of an eruption, Garces said. His group recently installed three detectors near the smaller erupting volcanic vents on the flanks of Kilauea, on the island of Hawaii.

"Each volcano has its own voice," Garces said. "A volcano that's quiet is purring, but when it's really ready to go, its infrasound voice gets louder and louder."

Of Elephants And Infrasound

One unusual but charming aspect of infrasound is that several species of animals use it to communicate, particularly in environments where visibility is difficult, as in the ocean or in dense forests.

Scientists have studied infrasound communication among whales and, more recently, land animals including giraffes, rhinos and elephants. Katy Payne, a Cornell University bioacoustics researcher, recently described what she observed among a group of elephants in Central Africa.

Payne first suspected that elephants might be using infrasound to communicate when she noticed "a throbbing in the air that I couldn't attribute to anything around me." She was always near the elephant cages when she felt the throbbing.

Biologists, she noted, had never studied the possibility of land animals making sounds below the frequencies people can hear.

"I thought maybe the elephants were generating that throbbing by making really powerful calls that were so low pitched that people couldn't hear them, " she said.

What she found is that elephants indeed have "an extensive vocabulary, including lots of sounds that are very rich in infrasound."

Low frequency sound travels much farther than sounds humans can hear. Perhaps, she wondered, infrasound is the basis for a long-distance communication system. That would explain all kinds of mysterious behaviors observed in elephants.

Payne and her colleagues are still investigating it, but she has described some of her field research on elephant infrasound in "Silent Thunder," a powerful plea for protecting the remarkable animals.

Infrasound At A Glance

Infrasound is a low frequency sound below the range of human hearing. It is produced by a variety of natural events - including earthquakes, volcanoes, avalanches, severe weather, ocean waves - and some man-made sources - including aircraft and explosions. It is also used by some animal species to communicate.

How is it different from audible sound?

One of the most interesting properties of infrasound waves is that they travel over global distances without losing their strength. So an instrument in Colorado can detect the sound of ocean waves hitting the coast in California or the roar of a typhoon halfway around the world.

How is it detected?

Infrasound wavelengths are so long that they can be detected only by sensors that take up a large ground area.

What is it used for?

A global infrasound sensor system is being deployed to detect nuclear weapons testing. Scientists are also studying ways to use infrasound to provide advance warning of natural disasters.