Great,
Thanks for the info.
N

On 13 Jul 2006, at 10:10, james fisher wrote:

> 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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