Establishing a sound connection with insufficient people....

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As the title says really.  Anyone got any tried and tested methods of testing for sound connections between caves when you've insufficient people to have somebody each side?  I'm thinking along the lines of...
1. Enter cave 1, leave device making noise and exit cave.
2. Enter cave 2 and see if you can hear it.

The 'device' will need to be loud (i.e. equivalent of someone shouting) and be able to make said noise for a couple of hours to give time to get between the locations.  It will need to be cave proof (but not waterproof) and not matter if I leave it for a week or two should I not bother to go back into cave 1 on the same day to retrieve it.  A bonus would be if the device also emitted light, though obviously leaving a torch shining is the obvious solution to this.  Ideally something dirt cheap.

All ideas welcome though I'm particularly interested to hear from anyone who's had success doing this sort of thing.

Cheers in advance.

I've seen this kind of thing used in the past.

They can make quite a bit of smoke though and probably aren't bat friendly.

Bob Mehew:
I would recommend snow avalanche transmitter & receiver, although the signal is radio rather than sound.  We managed to get proof of limited distance even though there was only solid rock between us.  And they kept going for days.  Otherwise possibly a rape alarm.  But you will have to mod it to put in a bigger battery to get the life time you want.  (That should not be to difficult and at £10 for one it should be worth a punt.)

I would hazard a guess that whatever you use to transmit will need to be low-frequency to have a chance of traveling through the rock - you may end up feeling it rather than hearing it, but that's fine. I've heard Hope Cement Quarry letting off a blast a few times inside Peak Cavern now, but I never heard the warning siren five minutes before!

If it wasn't for the possibility of leaving it in for a week, I'd suggest a boom-box playing drum n'bass. Alternatively, still leave a boom-box playing drum n'bass in, but also include some well-fed hippy dude and a UV light and tell him to keep changing the batteries until you return the next weekend ;)

A piezo sounder / buzzer, at about 2kHz (where human hearing is at its most sensitive). They take very little power, so a decent lead acid battery and it would  last for ages.



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