Echoes from the sky

Dotted along the Kent coast are the remnants of Britain’s wartime defences, some neglected and slowly crumbling away, but some renovated and given new lives. Today I decided to go down to Romney Marsh to see some of the most intriguing of these structures.

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martello towers

Before radar there were Listening Ears; Concrete Dishes; Acoustic Mirrors; now most commonly known as Sound Mirrors. These are huge concrete dishes were designed to amplify the sound of incoming enemy aircraft in order to give an extra 15 minutes early warning of an attack. There were built between 1928 and 1930.

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The mirrors are on a island, on private land (it’s owned by a gravel extraction company), and only open to the public on 2 or 3 days each year. In order to reach them you have to walk along a shingle path and cross a very narrow swing bridge.

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(you can just about see the mirrors in the distance)

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There are three designs of mirror; the first, a 20ft concave dish was constructed in 1928. It works by the ‘Listener’ standing on a platform in front if the dish with a ‘sound collector’.

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The second, completed in April 1930,  is a larger 30ft concave dish, with a concrete booth for the ‘Listener’ and equipment.

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The third, completed later in 1930, is the huge 200ft wall, which is a segment of a sphere. This much larger surface area enabled the ‘Listeners’ to pick up the sound from aircraft engines more effectively. Microphones were installed at intervals along a wall in front of the mirror. The ‘Listeners’ were situated behind the main wall.

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This large wall is particularly interesting because it is possible to test out its acoustic properties. I keep seeing people standing with their ears pressed to the wall and I did wonder what was going on, but then I got a chance to test it out myself with another visitor. When we stood some distance apart along the wall, both with our ears pressed against it, we could hear each other speaking in a normal voice, just like in the Whispering Gallery at St. Paul’s.

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I’ve been meaning to get down here to see them for ages, so I’m glad that I’ve finally managed it.

Just as a postscript, on the bus back to Folkestone, I actually spotted another small dish mirror up on the hill at Hythe. Apparently this is maintained (or not maintained) by the MoD, but it is a poor state.

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The next date for visiting the mirrors is 11th August, and I believe there’s also a date in November. These are well worth a visit. More details can be found on the website of the Romney Marsh Countryside Project http://www.rmcp.co.uk/NoticeBoard.php

Some of the details in this blog come courtesy of information provided by the Romney Marsh Countryside Project.

There’s a great aerial shot of the mirrors here http://www.greatstone.net/history/sound_mirrors.htm

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