Earth Speaks in an Inaudible Voice
You can’t hear it, but our planet’s ultradeep hum could save your life. No matter how closely you listen, you will not hear the Earth hum—but humming it is. Far, far below the range of human hearing, waves of energy are coursing through the crust, causing the ground beneath your feet to rise and fall about three-millionths of an inch every few minutes. First detected by networks monitoring seismic activity in 1998, the tiny ripples were initially chalked up to the many small earthquakes that occur each day around the world. But studies over the past decade have proved that the hum is far too constant for that explanation.
In February, oceanographer-turned-seismologist Spahr Webb, of
Earth’s hum turns out to be just one of the many enigmatic signals resonating in the range known as infrasound. Broadly defined as sound waves longer than 56 feet, infrasound lies below the rumbling 20-hertz bass notes at the threshold of human hearing. Hurricanes, tsunamis, and tornadoes also generate their own characteristic low-frequency noise.
Scientists are now studying infrasound to learn more about those powerful natural events and how to predict them—a skill that certain animals may already have. In fact, some animals generate their own ultrabass tones for a unique type of long-distance communication.
A big reason for the surge in interest is the growing number of monitors capable of picking up reverberations from nuclear tests, a mandate of the 1996 Comprehensive Nuclear Test Ban Treaty. In 2004, the stations helped scientists prove that what was feared to be a North Korean nuclear test was in fact the detonation of a train carrying explosives. The same basic technology can document a previously unheard world of natural sound as well. For example, physicist Alfred Bedard of the NOAA Earth System Research Laboratory in
The NOAA team has set up several arrays of sensors, spaced hundreds of miles apart, that register slight variations in air pressure—a sign of a lengthy sound wave passing by. (Scientists are not sure why tornadoes pump out infrasound, but Bedard thinks it may result from a rapid expanding and contracting of the vortex.) The readings are relayed to a laboratory in
Infrasound detection may also be useful for predicting volcanic events. Certain low-frequency activity may signal that a volcano is likely to spout an ash cloud, a column of jagged volcanic sand that can reach as high as 30,000 feet. An early warning would be tremendously useful, says
Infrasound would therefore seem to be a possible explanation for the folk wisdom that animals have a sixth sense attuned to impending natural disasters. After the 2004 tsunami, for instance, rumors spread of dogs that refused to go outside, zoo animals that wouldn’t leave their shelters, and elephants that burst their chains and rushed to higher ground.
Katy Payne, a bioacoustics expert at
Sitting near those elephants, she felt the same sensation. “I was feeling a throbbing in the air,” she says. “I guessed that the elephants were making sounds below the frequency humans can hear, and I turned out to be right. There was a whole frequency down there that nobody was paying attention to.”
Subsequent studies have shown that elephants generate infrasound calls to communicate over long distances. The elephants’ calls would be deafening—around 110 decibels, as loud as a rock concert—but we cannot hear a note because the waves are so lengthy, as long as 225 feet. This part of the sound spectrum travels especially well through water, so alligators, hippos, and whales use infrasound calls as well. The cries of blue whales may reach thousands of miles undersea.
But can infrasound-using animals actually sense a tsunami or other impending disasters? Caitlin O’Connell-Rodwell, who studies elephants at
This sixth sense is not the only infrasound-related conspiracy theory. Rumors persist that a low-frequency buzz pervades certain locations, allegedly inspiring foul moods or even suicides. In 2003, English artist Sarah Angliss built an enormous pipe capable of producing sounds below 17 hertz, then put on two concerts back to back. Two pieces in each concert included an infrasonic hum. After the infrasound-enhanced performances, she says, some listeners cited a feeling of coldness, anxiety, and shivers down the spine. Yet Bedard, who once carted his equipment over to
To Bass, these unknowns are what make the study of infrasound so exciting. “You hear things all the time, and 95, 98 percent of the time you haven’t a clue what they are,” he says. “They’re going to be equally cool once you figure them out.”
Kemo D. (a.k.a. no.7) www.beyondgenes.com