Denge sound mirrors

Spectacular remnants of a dead-end technology, the three concrete “listening ears” at Denge near Dungeness in Kent are the best known of the various early warning acoustic mirrors built along Britain’s coast.

[Photograph of the three sound mirrors at Denge]

[Dungeness sound mirror guided walk]

The only way of visiting the Denge sound mirrors is to go on one of the very popular guided walks run by the Romney Marsh Countryside Project. There is no public access to the Dungeness mirrors, which are in the middle of a disused gravel pit. Contact the Romney Marsh Countryside Project for the dates of future guided walks.

A forerunner of radar, the sound mirrors were intended to provide early warning of enemy aeroplanes (or airships) approaching Britain across the English Channel. They 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.

Aerial photograph of the sound mirrors at Denge

There are three different designs of sound mirror at Denge; the 200′, 20′ and 30′ sound mirrors.


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The 200 foot sound mirror

[Panoramic photograph of the 200 foot sound mirror at Denge]

[Photograph of the wall-style sound mirror]

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

[Photograph of the 200-foot wall-style sound mirror] [Photograph of 200 foot concrete listening ear sound mirror, 4 September 2005]
[Photograph of 200 foot sound mirror, 4 September 2005] [Photograph of the wall-style sound mirror]
[Photograph of 200 foot sound mirror, 4 September 2005] [Photograph of 200 foot sound mirror, 4 September 2005]
[Photograph of 200 foot sound mirror, 4 September 2005] [Photograph of 200 foot sound mirror, 4 September 2005]
[Photograph of 200 foot sound mirror, 4 September 2005] [Photograph of 200 foot sound mirror, 4 September 2005]
[Photograph of 200 foot sound mirror, 4 September 2005] [Photograph of 200 foot sound mirror, 4 September 2005] [Photograph of 200 foot sound mirror, 4 September 2005]

Old photo

[Historic photo of 200 foot sound mirror]
Thanks to Lawrence Mayes

The 30 foot sound 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 30 foot sound mirror, 4 September 2005] [Photograph of 30 foot sound mirror, 4 September 2005]
[Photograph of 30 foot sound mirror, 4 September 2005] [Photograph of the back of the 30' mirror] [Photograph of 30 foot sound mirror, 4 September 2005]

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 sound 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.

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

History

Echoes from the Sky by Richard N Scarth is the definitive book on the post-WWI mirrors, including the Denge mirrors and the Maltese one. Published by the Hythe Civic Society, it gives a detailed history of the sound mirrors and associated research projects, and is essential reading if you are interested in the story behind the mirrors. As Scarth points out on the guided walks, his book is based on detailed research and original primary sources, rather than stuff found on the internet.

Unfortunately Echoes from the Sky is now out of print, and Hythe Civic Society asks people not to e-mail them to try to buy a copy – they’ve none left. If you are interested in the mirrors and you ever see a copy for sale somewhere, grab it while you can.

Conservation

[Photograph of 20ft sound mirror at Denge taken by Paul Shearsmith]

July 2003: 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 were propped up – see Paul Shearsmith’s photo showing how the 20′ mirror had been undermined – and the mirrors 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

The mirrors are near Greatstone in Kent. An approximate grid reference is TR 075213.

[Dungeness sound mirror guided walk]

The only way of visiting the Denge sound mirrors is to go on one of the very popular guided walks run by the Romney Marsh Countryside Project and led Richard Scarth, author of Echos From the Sky, the definitive history of the sound mirror. The guided walks are 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.

Contact the Romney Marsh Countryside Project for the dates of future guided walks.

[Swing bridge to the Denge sound mirrors]

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!

If you visit the sound mirrors, feel free to add your photographs to the Sound Mirrors pool at Flickr.

Place names

[Photograph of sound mirrors, 4 September 2005]

The location of the mirrors is described in a number of ways depending where you look. I’ve gone for Denge, as it seems popular, and is used in assorted (quasi-)official sources including historic quotations in the book Echoes from the Sky.

Peter Faulkner of The Greatstone Website say: For the record, the Sound Mirrors are physically sited in Greatstone on an island on Greatstone Lakes. Dungeness is some three miles way to the south (albeit it is accurate to say that the Mirrors are in the Dungeness National Nature Reserve). The area known as Denge Marsh or Dengemarsh [there is no place actually called Denge] starts about a mile to the south east and continues southwards. Lade is not a place but a name taken from an old Napoleonic fort called Lade Fort located about a mile to the south east of the Mirrors in Lydd-on-Sea.

Links

  • The Romney March Countryside Project website has short history of the three sound mirrors.
  • Dungeness National Nature Reserve:

    At the back of two gravel pits at Lade on an island are the three concrete listening mirrors, built in the 1920’s and 1930’s to detect enemy aircraft as they approached Britain. This is the only site in Britain where all three designs are situated in one place. This early warning system with a range of 20 miles became obsolete by the outbreak of the Second World War, but they have survived and are popular with visitors on pre-arranged guided walks in the summer.

  • The Denge sound mirrors were the 20th Century Society’s Building of the Month for October 2003. “Twentieth century lumps of concrete have only recently been recognised, even amongst English Heritage”, said Kendall. Did nobody tell them concrete was cool?!
  • 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.

Music

More information

News and updates about the Denge sound mirrors.

There is a similar 200ft mirror at Maghtab in Malta.

The Denge mirrors are the most famous, but there are a number of other early warning acoustic mirrors in Britain.

If you visit any of the sound mirrors, do add your photographs to the Sound Mirrors pool at Flickr.


[Photograph of the three mirrors]

Sound mirror at Denge near Dungeness in Kent./td