Denge sound mirrors

Spectacular remnants of a dead-end technology, the three "listening ears" at Denge near Dungeness in Kent are the best known of the various acoustic mirrors built along Britain's coast. A forerunner of radar, the sound mirrors were intended to provide early warning of enemy aeroplanes (or airships) approaching Britain.

The mirrors did work, but the development of faster aircraft made them less useful, as an incoming aircraft would be within sight by the time it had been located. Increasing ambient noise made the mirrors harder to use successfully, and then radar rendered acoustic detection redundant.

There are three different designs of sound mirror at Dungeness;
from the left are the 200', 30' and 20' mirrors.

The 200 foot mirror

[Photograph of the wall-style sound mirror]

This is the biggy. There is another similar mirror at Maghtab in Malta.

[Photograph of 200 foot sound mirror, 4 September 2005]

[Photograph of the 200-foot wall-style sound mirror]

Old photo

Thanks to Lawrence Mayes

The 30 foot mirror

[Photograph of the 30' mirror by Paul Shearsmith]

The 30' dish still has the metal pole which a microphone would have been fixed to. The mirror worked by focusing the noise of aircraft engines onto the microphone, which amplified the sound. The relatively slow aircraft of the time could be heard and located before they came into sight.

[Photograph of 30 foot sound mirror, 4 September 2005]

[Photograph of the back of the 30' mirror]

The height of the top of the flight of concrete steps down into a chamber below the dish show how far the ground level has fallen since the mirror was built.

The 20 foot mirror

[Photograph of 20 foot sound mirror, 4 September 2005]

The 20 foot mirror at Denge is a bit closer in design to the First World War period acoustic mirror at Kilnsea in East Yorkshire.

Conservation

[Photograph of 20'sound mirror by Paul Shearsmith]

July 2003: Some very good news. Britain's Concrete Ears To Be Saved By English Heritage. English Heritage used a £500 000 grant from the Aggregates Levy Sustainability Fund to help stabilise and preserve the mirrors. Local authorities secured £125 000 to provide visitor interpretation as part of the Historic Fortifications Network.

The foundations will be propped up - see Paul Shearsmith's photo (right) showing how the 30' mirror had been undermined - and they will be put on an island to protect them from the morons who seem to enjoy physically destroying historic structures and covering them in graffiti.

Location and visiting

At present the only way of visiting these mirrors is on one of the very popular guided walks run by the Romney Marsh Countryside Project and led Richard Scarth, the author of the book Echos From the Sky. I went on the walks in September 2003 and September 2005, and it is well worth doing. The mirrors really are impressive close up.

Other than the guided walks, there is no public access to the Dungeness mirrors, which are in the middle of a disused gravel pit. It used to be possible for trespassers to gain access to the mirrors, but this is no longer physically possible. The deep lake created by gravel extraction has been extended to completely surround the mirrors, which are now on an island. The only access to the island is via a swing bridge, which is locked in the open position when not being used for guided walks. The good news is that this should keep out vandals and undesirables!

[Photograph of sound mirrors, 4 September 2005]

An approximate grid reference is TR 075213.

Links

The 20th Century Society's October 2003 Building of the Month.
There are some photos of the listening ears on the Pilot pub's website.
In June 2001 The Guardian had an article on a proposed art project using the mirrors: This project, though, thematically twists the mirrors by 180 degrees, turning what was to be a shield of defence and surveillance into a tool for communicating with the continent. A new sound mirror will be built on Dungeness and a second facing it near Boulogne, probably at Wimereux ... So when you stand at a certain point in the mirror, you'll be able to hear the voice of the person standing by the mirror in France - but only at an exact spot in the mirror. Move just a centimetre and the sound will disappear.
More acoustic mirror based art proposals, with some interesting pictures of the mirrors.
Justin Bennett's website has a scan of a newspaper article on the Lade wall.
A photograph of the wall by Jon-Paul Mountford.
The band Turin Brakes used pictures of the Denge mirrors on the covers of their CDs Ether Song and Long Distance.
... as did Blank & Jones on Monument and the video A Forest.