Bob Mehew said:
he perhaps erroneously mentions in the title of the thread but to correlate
Ah yes, my apologies. I certainly wasn't trying to suggest you could go directly from dB of noise --> flow rate with no other information. But I do still suspect that noise power level will be proportional to flow rate, though quite possibly with nasty non linearities and transitions. Which is a complicated way of saying that 'if the pitch sounds more noisy, more water is falling down it'.
Price is a certainly a strong factor, as this is entirely for my own entertainment!
It certainly seems possible to put together a data logger with accurate barometer, temperature and microphone for a parts cost of around ?30-40, based around the Arduino / Atmel AVR platform.
A friend also suggested the really cute idea of getting it to record a segment of ambient cave noise at Midnight on the new year.
I think an easy way of building a 'propeller' anemometer would be to start with a large (new!) brushless computer fan, then directly tap the coils / use the hall effect switches built into it. The problem is that cave wind speed is so low that you have very little force at all to deal with, you'd need to find something with very good bearings to have it spin at low flow rates.
The good thing with a fan is that it's easy to see which way the flow is! With a hot wire anemometer, you would probably need a second hot wire purposefully shadowed from the wind in one direction.
Still, I honestly think that with a bit of design and engineering you could build a little data logging box that'd be able to extract a lot of interesting information including audio remote sensing, and do some serious science. I'd certainly be interested to know how the cave draught varies with weather systems + seasons, and understand a little more of the hydrology + nature of rain driven flood pulses vs. yearly snow melt.