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I may be way off here but, I thought the Helmholtz resonator is a band stop filter. So it is used to keep noise down, I used this stuff when making big sub woofer boxes. See this page for some equations; http://www.gmi.edu/~drussell/GMI-Acoustics/Filters.html I hope this helps and of course I...

This may be a stupid idea but what about that conductive tape that alarm companies put on glass to tell if the glass has been broken, you could put a couple of small pieces together and if water was present it could complete the circuit like a switch.