Let me just ask the forgiveness of all the golden ears among this group
right up front--as well as from Eric and the other technical types....
But, this seems like a perfect time to just let something cheap and
crude work for you. I would guess that you could glue a large-diameter
piezo bender onto the end of a PVC pipe cap and wrap it with aluminum
foil for shielding. Gut a cheap electret capsule for the FET,
connecting the backplate to the silver electrode on the piezo and what
used to be the case of the electret to the bender plate of the piezo and
your shield, then plug the thing into your recorder's PIP mic input.
Or, if you have better low-frequency response using your recorder's line
input (and you probably will), make an ultra simple preamp out of a
cheap TL08x op amp. DC couple it if you can or use big fat coupling
capacitors. I'm obviously not trying to supply a design here--just the
idea that you could make something easily for a few dollars.
You will be working with frequencies that are well below the resonance
of any materials that you might use in your DIY infrasound mic, so it
should be pretty linear in the range of infrasound as long as you have a
buffer circuit with a high enough input impedance. And it probably
doesn't need to sound that great (not that it couldn't) because you
can't hear it anyway. This thing would be exceptionally sensitive to
handling noise and wind. Peizos are also pyroelectric, so rapid
temperature changes would also saturate your preamp. But these problems
are probably inherent in scope of infrasound recording, as has been
mentioned previously.
On another note: I was wondering if strong infrasound could be the
result of some Hemholtz resonance (like the sound made when you blow
across the top of a beer bottle) created by the wind blowing into a
canyon or cave from just the right direction.
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