by Charles Q. Choi

Special to LiveScience
16 April 2008

from LiveScience Website



Earth gives off a relentless hum of countless notes completely imperceptible to the human ear, like a giant, exceptionally quiet symphony, but the origin of this sound remains a mystery.

Now unexpected powerful tunes have been discovered in this hum. These new findings could shed light on the source of this enigma.

The planet emanates a constant rumble far below the limits of human hearing, even when the ground isn't shaking from an earthquake. (It does not cause the ringing in the ear linked with tinnitus.) This sound, first discovered a decade ago, is one that only scientific instruments - seismometers - can detect.


Researchers call it Earth's hum.

Investigators suspect this murmur could originate from the churning ocean, or perhaps the roiling atmosphere.


To find out more, scientists analyzed readings from an exceptionally quiet Earth-listening research station at the Black Forest Observatory in Germany, with supporting data from Japan and China.



Different types

In the past, the oscillations that researchers found made up this hum were "spheroidal" - they basically involved patches of rock moving up and down, albeit near undetectably.

Now oscillations have been discovered making up the hum that, oddly, are shaped roughly like rings. Imagine, if you will, rumbles that twist in circles in rock across the upper echelons of the planet, almost like dozens of lazy hurricanes.

Scientists had actually expected to find these kinds of oscillations, but these new ring-like waves are surprisingly about as powerful as the spheroidal ones are. The expectation was they would be relatively insignificant.



New thinking

This discovery should force researchers to significantly rethink what causes Earth's hum.


While the spheroidal oscillations might be caused by forces squeezing down on the planet - say, pressure from ocean or atmospheric waves - the twisting ring-like phenomena might be caused by forces shearing across the world's surface, from the oceans, atmosphere or possibly even the sun.

Future investigations of this part of the hum will prove challenging, as,

"this is a very small signal that is hard to measure, and the excitation is probably due to multiple interactions in a complex system," said researcher Rudolf Widmer-Schnidrig, a geoscientist at the University of Stuttgart, Germany.

Still, a better understanding of this sound will shed light on how the land, sea and air all interact, he added.

Researcher Dieter Kurrle and Widmer-Schnidrig detailed their findings March 20 in the journal Geophysical Research Letters.








What Makes My Ears Ring?

from LiveScience Website

So you've rocked out at a summer's worth of heavy metal music festivals, and now you want to know why you still hear feedback.

That ringing or buzzing in the ear is likely a symptom of hearing loss resulting from excessively loud noises. But tinnitus (the perception of a sound without external stimuli) can also accompany infections, earwax buildups, vertigo, large doses of aspirin, and even a tumor on the acoustic nerve.


Of course, your phantom buzz might indicate nothing more than aging ears. Experts aren't sure what physiological process creates the noise, only that it chimes in for a number of afflictions.

If you suffer chronic tinnitus, try not thinking about it! Clinical observations suggest that simply focusing on unrelated thoughts can reduce activity in the auditory cortex and lessen the perceived loudness.


Oh, and you might skip Ozzfest next year, too.







Source of Earth's Hum Revealed, Space Symphony Possible
by Robert Roy Britt
Senior Science Writer
26 March 2000

from Space Website

Competing with the natural emissions from stars and other celestial objects, our Earth sings like a canary - it drones on in a constant hum of a gazillion notes.


If it were several octaves higher, and hence, audible to the human ear, it could probably drown out the noise from a hundred TV talk shows.

In recent years scientists have used seismographs to sort out these subsurface sound waves from earthquakes (all seismic waves are, essentially, the in-ground equivalent of sound waves). But what causes the hum, which researchers call the background-free oscillation, has been a mystery.

The apparent answer, revealed in the March 24 issue of the journal Science, is as surprising as the hum itself.

Kiwamu Nishida of the University of Tokyo's Earthquake Research Institute has, along with colleagues, analyzed 10 years of seismic data and tied the seismic waves to similar oscillations in the atmosphere.

Inaudible sound waves in the lower atmosphere push and pull on the ground, the researchers say, creating coupled "sound" waves, or seismic waves, inside Earth. The initial source, as yet not determined, could be changes in atmospheric pressure. The researchers also did not rule out possible oceanic sources, such as pounding waves, as the cause of Earth's hum.

The strange-but-true solution was first proposed in 1997 by Naoki Kobayashi, a theorist at the Tokyo Institute of Technology and co-author of the new paper.


A space symphony?

Because Mars and Venus are both solid bodies with atmospheres, Nishida told that our two nearest planetary neighbors are probably humming too, creating a miniature symphony in space.

"Because the density of these atmospheres is different, amplitudes of the 'sound' might be different," Nishida said. "On the other hand, the amplitudes [within the planets] might be similar to that of the solid Earth."

The sounds are below 10 millihertz, whereas 10,000 millihertz is about the lowest audible to the human ear. Which means you can't hear the hum.


Good thing, because the discordant sound has been described by one geophysicist as "a very messy noise."


The sound of Mars

Philippe Lognonne of the University of Paris said a proposed future mission to Mars, known as NetLander and expected to launch between 2005 and 2007, would explore the seismic waves of the Red Planet and their possible connection to the atmosphere.

"We expect this signal to exist also on Mars, and to detect it will therefore give us the possibility to see the free oscillations of Mars," Lognonne told


"It is clearly a technical challenge, but it might be possible."

The NetLander mission would use a network of seismic detectors to measure Mars-quakes - theorists expect the gradual cooling of the planet generates 50 or more temblors a year with magnitude 3.5 or greater. Studying how the seismic waves move through subsurface layers of different composition would help scientists determine the diameter of the planet's core and whether it is solid or liquid.

A set of four meteorology stations would monitor the martian atmosphere and how it interacts with the surface.