Sound detectors

I've been trying to find out about these things ever since I stumbled across a magnificent photograph in (I think) an old 1920s Proceedings of the Royal Institution. The photograph showed a chap sitting on a converted anti-aircraft gun mount, two massive ear-trumpets jutting out, with stethoscope tubes plumbed between them and his ears. Gloriously insane, and apparently the best way of detecting inbound aircraft in the pre-RADAR days.

Now, I did know that scattered up and down the East and South coasts stand the remains of sound mirrors. But I didn't know there were so many, until this post on Metafilter this morning. Denge looks like it's worth a visit, while this page has pictures of devices more like the one I remember.

So now I know where these things are, I simply have to dream up an excuse for filming at one. The only trouble is, building something like this strikes me as, potentially, hideously dangerous. Unless one introduces electric microphones and amplification -- which feels like cheating -- there's no prospect of noise limiting, which could really ruin your whole day.