Hi, a piezo disk will pick up the higher frequencies
you are interested in. use a 100 gram weight on top of
the piezo disk and bury in a 1' deep hole to isolate
disc from direct sound pickup. Superglue a strong
magnet to disk and stick on a iron rod driven into the
ground. Insulate everything well. See past posts on
piezo disks. They really work well for picking up
seismic waves as well as sound waves.

Besr Regards

Jim

--- Mark Robinson 
wrote:

> Nicholas Ward wrote:
> > Hi,
> > I realise this is probably not the kind of request
> you get everyday and 
> > would like to say thanks in advance for any advise
> you folks might be 
> > able to offer.
> > 
> > Im a multimedia artist working here in ireland. I
> have an installation 
> > which will be part of a music festival here in
> september.
> > 
> > http://www.electricpicnic.ie/flash.html
> > 
> > For the installation i have proposed to capture
> the sound of 10000 or so 
> > people all jumnping in unison to the music and
> play it out through small 
> > speakers situated at the far end of the festival
> from the main stages. i 
> > also anticipate picking up some of the music as it
> is transmitted into 
> > the ground via the mainstage (if he had ears)
> would hear under the 
> > festival.
> > 
> > I was thinking a geophone would be the right
> device to capture the sound 
> > with. Im most interested in the 20 to 1200Hz range
> of frequencies.
> > 
> > I have no experience with this type of equipment
> and was hoping you 
> > might be able to advise me on a suitable device
> for this project? Also 
> > what type of amplification would i need to bring
> the output signal up to 
> > a line level, (around 100mV).
> > 
> > Any advise such as whether a geophone would be
> sensitive across this 
> > frequency range, where i might get one, how much
> they cost etc that 
> > could be offered would be really great.
> > 
> > Many thanks
> > 
> > Nicholas
> 
> Hi Nicholas,
> 
> Geophones are well suited to your application
> although there are some pitfalls.
> 
> They would be unlikely to capture much above about
> 100Hz. Ground doesn't 
> transmit such frequencies well anyway.
> 
> A geophone will do a good job of capturing
> vibrations in the ground and yes the 
> effects are interesting through a *large* PA. It
> gives the whole venue more 
> "feel". You can connect it straight into the
> microphone input of a sound desk 
> and get excellent results.
> 
> Small speakers will not give good results as the
> frequencies you are dealing 
> with are very low.
> 
> You could feed the geophone signal through a music
> synthesizer to generate a 
> signal in the audible spectrum.
> 
> Most oil exploration companies and university
> geophysics departments will have 
> some. Larry's for sale list at
> http://psn.quake.net/geophone/index.html seems 
> to say he has none in stock.
> http://www.tenrats.org/geo.shtml lists possible 
> sources.
> 
> cheers
> Mark
> 
>
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