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.

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

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

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The 200 foot sound mirror

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

Old photo

Thanks to Lawrence Mayes

The 30 foot sound mirror

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.

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

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

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.

The only way of visiting these mirrors is to go 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. The 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. 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!

Place names

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

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

Music

• 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.
• … while The Prodigy included the mirrors in the video for Invaders Must Die.

More

News and updates about the Denge sound mirrors.

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

Sound Mirrors pool at Flickr.