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I would like to create a very simple percussion instrument that would match the resonant frequency of an echo-y cave I occasionally visit. Currently I am thinking of using a metal bar, but I don't know how I would tune it or what metal I should use. I am not a musician. Does anyone have any ideas?

Thank you!

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2 Answers 2

up vote 5 down vote accepted

Based on what you've given us to go on, the instrument with the resonant frequency of this cave you visit would be an identical cave. :-)

Before you start thinking about instruments, materials, or tuning, you need some ballpark as to what this resonant frequency actually is.

The most reliable way to determine this would be to bring a tone generator (like a laptop) and amplifier into the cave, and sweep through a broad range of frequencies until you find the one that hits. But you don't have to go to those lengths if you have some information to start with:

  • Can you hear a resonant frequency when you are in the cave, perhaps from the sound of wind? If so, then the frequency is within the range of human hearing, i.e. 20Hz-20kHz.
  • Can you match the resonant frequency using your voice? If so, then the frequency is within the range of the human voice, approx. 80Hz-1,100Hz

Once you have your target frequency, then you can go about finding/tuning an instrument. Honestly, in the percussion world of found instruments, this is mostly just trial and error. If you are making your own instrument, there are simple formulas for the resonant frequencies of a cylindrical tube, which may be useful.

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+1 Using that, OP should be able to roughly tune, say, a length of PVC pipe. –  luser droog Mar 2 '13 at 6:10
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Exactly, assuming the target frequency is high enough! It might be a bit impractical to go spelunking with a 10-foot tube. :) –  NReilingh Mar 2 '13 at 6:39
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The thing you haven't really taken into account is that all instruments (including metal bars) have a range of frequencies, not just one, and you may have a range of resonant frequencies in the cave itself, so you will need to think about what tone you are intending to get.

For example you may use an instrument that happens to have a harmonic at a resonant frequency of the cave, which means the tone of the instrument will sound very different inside and outside the cave - as the harmonic may be accentuated over the root frequency - which can give some excellent tonal changes.

Metal rods and plates can be pretty difficult in this respect, as the distribution of harmonics is not necessarily as obvious as those on a string or wind instrument.

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