FolkestoneJack's Tracks

A trio of sound mirrors

Posted in Dungeness, England by folkestonejack on July 15, 2017

The other worldly landscape at Dungeness holds many surprises, having been the home to many an experiment in new wireless, radar and navigational technologies. The most curious of these is a trio of ‘sound mirrors’ dating to 1928-30 which were at the cutting edge of advances in the detection of aircraft by sound long before they became apparent to the unassisted human ear.

Although the technology was proven it would never see full operation, having been rendered obsolete by developments in the use of radio waves that would go on to become radar. In some ways this short-lived field of military technology is all the more fascinating for its unfamiliarity, whether that be these monstrous fixed concrete listening ears or the even more extra-ordinary mobile sound locators.

The three sound mirrors are on an island that can only be reached by a narrow swing bridge used for open days and guided walks

The 20ft, 30ft and 200ft sound mirrors that survive at this site are now surrounded by the deep-waters of the Greatstone Lakes, formerly the Lade Pits, which are man-made quarries that have steadily been reclaimed by nature since the end of sand and gravel extraction. Today, the site forms part of the RSPB Dungeness Nature Reserve.

The three sound mirrors present a perfect demonstration of the development of the technology. The earliest of these, the 20 foot sound mirror, was completed in July 1928 and would have been operated using a swing-able collecting trumpet connected to the listener by a tube and stethoscope.

The 30 foot mirror, constructed between February and April 1930, afforded its operators a little more shelter with a listening chamber enclosed with glass windows. This sound mirror also used a swing-able collecting trumpet and this mechanism is surprisingly intact (with the exception of the trumpet that sat at the very end). A look at period photographs shows that the ground level has dropped by at least 3-4 feet here, exposing concrete that would have sat well below the single at the time of its use.

The decision to go ahead with the construction of the largest of the sound mirrors, a 200 foot long curving concrete wall with a height of 26 feet, took place in late 1928 and work was complete by the summer of 1930. This strip mirror was intended to pick up the low frequency sound waves from approaching aircraft at three times the range of the unaided ear (if not more) and could be used to identify the bearing and distinguish aircraft (so long as they were separated by more than 10 degrees).

This giant introduced a number of advancements, making the move from a swinging trumpet-shaped sound collector to a series of 20 microphones, placed five foot apart on the concrete forecourt. At first the monitoring was carried out from a hut beside the wall, but in 1933 a control room was added behind the mirror with a window was cut into the concrete to give the operator a clear view of the entire forecourt.

The 200 foot strip mirror was built to endure the strongest winds, with steel-re-inforcement bars inside the wall and buttresses at the rear every 10 feet. This was also sufficient to ensure that attempts to demolish it in 1940 proved trickier than expected, leading to the abandonment of the attempt (the initial focus of the demolition was on the buttresses and the damage caused by this was subsequently rectified during a restoration programme in 2003). Instead, the mirrors were abandoned and left to crumble in their own time.

The 20 and 30 foot sound mirrors at Greatstone

I was interested to read that the experimental establishment had problems with inappropriate access as early as 1930, with staff having to turn away day trippers. This necessitated the replacement of boundary stones with a barbed wire fence. It is testament to the continued curiosity factor and appeal of the sound mirrors that this has remained a problem all of their life.

The shifting and sinking shingle continued to expose the foundations of the sound mirrors, leaving them in a perilous condition. The future might have seemed bleak but it was at this point that English Heritage stepped in with a restoration project partly funded by the Aggregates Levy Sustainability Fund. The marvelous sight of the trio, now stabilised and in a much improved condition, is testament to the success of that work.

Practicalities

The Sound Mirrors are usually only available to see close-up on a handful of occasions each year, for open days or guided walks. If you haven’t managed to time your visit for one of these you can still get a pretty decent view of the mirrors from the pathway alongside the Greatstone Lakes. It’s worth keeping an eye out on the event listings of the RSPB Dungeness Nature Reserve and the Romney Marsh website.

Footpath across the shingle

To get to the site of the sound mirrors I caught a train down to Ashford, Kent, and then picked up a number 11 bus towards Lydd-on-Sea, getting off at the bus stop at Coast Drive/The Parade nearest to Derville Road (alternative stops at Taylor Road and Seaview Road would work equally well) after a ride of just over an hour. Another option would be the 102 from Folkestone. There are three entrances to the nature reserve at Lade Pits – I took the option that I thought involved the least trudging across shingle, walking up Derville Road, taking a right into Leonard road and then up a pathway between the houses to a gate into the reserve (I’m not sure if this was opened specially for the open day – the public footpath from Taylor Road, which I used on my way back, could be a safer bet if you are walking to get the view of the sound mirrors from across the lake).

At the immediate left-hand side of the gate are the remains of a small bridge that took the Southern Railway’s re-aligned standard-gauge line over the top of the narrow gauge War Department branch line of the Romney, Hythe & Dymchurch Railway and on to New Romney. The standard gauge line originally ran behind the sound mirrors but was re-aligned to serve the holiday camps established on the coast. It only lasted thirty years, from 4th June 1937 until 6th March 1967.

The short narrow gauge War Department branch line was equally short-lived, lasting from 1929 until 1951. It had played a crucial role in the story of the sound mirrors, carrying a great deal of the material needed for its construction, before ending its life serving freight traffic from the quarry.

Turning right after the gate you follow a footpath along the perimeter of the lake, on what was once the trackbed of the now dismantled Southern railway line, until you come to a loose shingle pathway on your left after a short walk. Taking this turning leads you towards the centre of the lake and to the very narrow bridge across to the island (where the original causeway was cut to create a barrier to deter trespassers). Most of the year this is locked out of use, preventing access to the island, but on open days you can walk across one foot at a time (when they say narrow, they really mean it!). The effort is well worth it – the sound mirrors really are stunning close-up and this view really allows you to appreciate details invisible from a distance.

It is well worth picking up a copy of the excellent book ‘Echoes from the Sky‘ by Richard Scarth (now available in a revised and expanded new edition, published by Independent Books in 2017). This meticulously researched work presents the fullest account of the development of the sound mirrors, drawing on original sources and private papers. Along with the wonderful photographs of the sound mirrors under construction and in operation that are included in the book this account really helps you understand what you are looking at. I can’t recommend it highly enough.

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  1. […] Hythe and Dymchurch narrow gauge War Department branch line (as mentioned in my recent post ‘A trio of sound mirrors‘) prompted a few memories of the operation of the standard gauge line. I thought it was an […]

  2. […] The piece from Marc Schmitz and Dolgor Ser-Od really appealed to me as it draws on the ‘listening ears’ along this stretch of coast which I just happened to have visited a little over a month back. […]


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