Charles Q. Choi
Special to LiveScience
16 April 2008
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
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.
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.
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
"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?
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
Oh, and you might skip
Ozzfest next year, too.
Source of Earth's Hum Revealed, Space
by Robert Roy Britt
26 March 2000
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
SPACE.com 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
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
The sound of Mars
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
Red Planet and their possible connection to the
"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 SPACE.com.
"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.