An interesting site showing what was in use before radar. I have never seen or heard of these. There was also a bigger one set up in Malta. http://www.ajg41.clara.co.uk/mirrors/index.html
I saw these on Map Man a couple of years ago. One still stands in the marshes of the Wash (I think) in the centre of a nature reserve. Listed structures all of them. i think there were a couple of mirrors and a large arced wall too. They actually tested them and it did enable them to hear a Tiger Moth coming before seeing or hearing it naturally. Quite simple things. But Watson Watt came along and they became history.
A slightly better picture than the one on the link, of the Sound mirror at Kilnsea in Yorkshire, dating from 1917. They had some degree of usefulness in 1917-18, providing the enemy conveniently flew within a few miles of the mirror (we're talking aeroplane bombers not airships by this time), and prviding there was no background noise (so they were situated in isolated areas). The pole just visible in from the bowl held a microphone, which was pointed at the part of the mirror which gave the loudest sound and hence the direction of the enemy could be calculated (roughly). But sound-waves travel very considerably slower than light or radio (electro-magnetic) waves, so any usefulness the mirrors had against Gothas traveling at 90 mph, giving maybe 20 minutes or so for the Home Defence fighters to intercept, had dissappeared by the time of WW2 when the enemy was travelling at 250mph, giving maybe four minutes warning.
Not only were there giant sound mirrors, but there was a smaller sound locator. Here is a photo of one in action and is taken from the book, Military Science Today By Lt Col Donald Portway RE from 1940
