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An Isotropic Earth?

by Jim Sorenson

We were talking about something along this line on the regular old Syn-Aud-Con website list-serve the other day when it occurred to me that this might be the place to write a line or two.

We were talking about infra-sonic sound…really low frequencies…and the effect thereof on how we behave.  I mentioned that I had once been hired to measure very low frequency vibrations associated with punch-press operation and how it’s very intense ELF component had potentially caused chest damage in some too close to the noise source.

That got me to thinking about isotropic radiators, which of course must expand and contract in order to produce the audible or measurable result.  I guess we’re all used to the notion of a loudspeaker cone or diaphragm moving forth and back (think about it!) but seldom think about the idea of an expanding, contracting spherical “membrane” as a sound source.  Of course it’s not going to be a perfect “isoptrope” because those things are essentially mathematical concepts which don’t exist in real life.  Very much like a Congress that votes for a tax decrease.

I’m trying to picture the earth, which is an elastic ball about 8,000 miles in diameter, vibrating at some frequency and…even scarier but more interesting…having a resonant frequency!  Ooooh!

We know that the earth gives off audio in the form of atmospheric disturbances from fault realignment…earthquakes…of which there are several thousand measurable a year and a couple of hundred noticeable.  Every thousandth of a percent or so we get a real knee slapper that slings you our of bed and might darned near wake up your spouse.  That’s actually happened to me but I’d rather not get into details.  Publically.

I guess the question is why hasn’t someone invented an isotropic loudspeaker?  We have the dodecahedron type that are used for measurements but not much else probably because they don’t sound all that great when you try to play music through them and that might give us our answer.  There are a couple of manufacturers who make “spherical” loudspeakers but they’re really only a ball with a conventional loudspeaker using it as some sort of baffle.  They may be very omni-directional but only in one mode.

While we like the idealized notion of sound coming from legitimate point sources…example from each and every individual violin in an orchestra…we probably wouldn’t want to listen to it that way.  We’d hear the individual differences in the instruments which would probably be pretty aggravating rather than simply the blended musicality of the orchestra section.  What’s interesting is that a single violin sounds kind of thin in an orchestral setting…unless playing a solo…but the whole section sawing away is both artistic and impressive.  On the other hand a single triangle tinging in the percussion section sounds just fine but a triangle ensemble would sound awful.

Another consideration in making an isotropic loudspeaker would be the fact that it has a Q of 1 and can’t have any other Q, no matter what frequency.  A sphere is a sphere and can’t and doesn’t behave like anything else.  That being true then the only thing that would impact the operation of the thing would be friction from expanding and contracting more and more rapidly with increasing frequency.  Of course that would generate an increasing amount of random chaos…heat!…which might well lead to something like spontaneous ignition or perhaps an explosion.  Neither of which, while admittedly entertaining, is consistent with good sound.

The other thing with a Q of 1 would be that you’d have to put in a bunch of power to get a relatively small output signal.  the output of the total sphere would be the power less the losses (friction again) but in any given rotated “cardinal” direction would be something less than 8%.  Poor choice for a baseball park.

Finally we’d get to the point of how do you make stereo?  I don’t even want to go into that!  Probably a situation where equidistant between the needed two radiators you’d have a zone with nothing measurable but an SPL so high it’d kill itinerent mosquitoes.  We have interferences “irises” in antenna work so why not the “iris of murderously loud sounds that you can’t hear” in isotropic loudspeaker systems?

Personally, I’m happy with the idea that the earth while it’s spinning on it’s merry way around and around the sun is vibrating under the influence of some pretty serious tidal forces from the sun and the moon as well as the forces caused by the multi-speed rotation of the core, the mantel, and the surface junk.  Getting back to the original question, does this ELF noise affect us?  My guess is probably.

I know that animals like pigs are very much affected by whatever the precursor of an earthquake feels or sounds like, as are some dogs.  They tell me that horses are also thus affected.  Things like seismographs and people, not so much, which is kind of interesting.  What is it that’s affecting these animals but not affecting our technology or ourselves?  We’re animals and we’re very much liked pigs anatomically, at least so says my doctor.  Of course he could be referring to girth and not to where I keep my kidney’s.  Then again, his mother is English and his dad’s Chinese so you can’t trust him on two counts.

Clearly the technology used in seismic detection is very mature so I think if’ there’s a vibration, the seismograph will seismogrify it and we know these disturbance aren’t electrical or magnetic in nature so that seems to leave “sound.” “Infra-sound” at that.  Are pigs hearing things we humans can’t?  Maybe those “hornlike” ears are the ticket.  It would explain Mr. Spock but it leaves the question of whether Orwell was right after all?

As far as creating a signal that would be the resonant frequency of the earth, I think I’ll leave that to Ming the Merciless!

Keep it out of the red!

Jim Sorensen