Earth's Constant Hum Explained

MattSparkes writes "It has been known for some time that there is a constant hum that emanates from the Earth, which can be heard near 10 millihertz on a seismometer. The problem was that nobody knew what caused it. It has now been shown that it is caused by waves on the bottom of the sea, and more specifically 'by the combination of two waves of the same frequency travelling in opposite directions.'"

Comments

Did ancient greeks know about this?

[torrija]

I think this is a concept related to Pythagoras' Musica Universalis http://en.wikipedia.org/wiki/Musica_universalis. An inaudible sound on all celestial bodies.

But wait!

[camperdave]

...the waves are making the sound.

Wait a minute. How do we know that it's the waves that are causing the hum, and not the other way around? Perhaps the planet is still ringing from meteor impacts, and the hum is just the resonant frequency. The deep ocean waves may be just a side effect.

I have an idea

[LaughingCoder]

Maybe we could build a clock that used this hum as some sort of synchronization. Then every clock on the planet could be synchronized, since this signal is presumably detectable everywhere.

OK, I didn't say it was a *good* idea :-)

Not quite right

[unixfan]

All objects have a mean frequency which in this example is causing the frequency that they are observing in the water. The frequency in the water comes from the planet's own resonance, or a harmony thereof.

Tesla noticed this and build a little tool which hit on the harmonic frequency and kept accelerating the oscillation with a device he built until there were "earthquakes" observed all around, and he had to cut short a trip to run home and turn it off. Indeed in manufacturing speakers you try to get this frequency down below audioble range as you don't want the speaker to resonate and alter the sound it's supposed to generate.

It's a very common mistake made by many when they observe a symptom (not realizing there is a real why behind it.)

this is what I found

[mapkinase]

Article reporting the milliHz hum in 1998
IDA (International Deployment of Accelerometers) used to detect the hum.
Article in Nature (1979) assesses if IDA can be used to detect very low frequency seismic data. Looking at the figure 1 of amplitude(?) ("MD counts" at Rarotonga station not shown on the current IDA map) I can see the aftershocks in 2 hour intervals after the Indonesia earthquake, but the subj frequencies could be detected only by obtaining the spectrum (Fig.2) at mHz range which frankly looks like white noise - irregular beats.

Most interesting figure is Fig.3 which shows the 0.43-0.52mHz of the _processed_ spectrum measured at six different stations around the world at Hour 25 and on. The Alaska station (CMO) has much clearer spectrum compared to the closest (?) RAR station.

All of it must have meant something for a seismologist which I am not.