GREATSTONE SOUND MIRROR SYSTEM

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GREATSTONE SOUND MIRRORS

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The site of three sound mirrors at Denge. Sound mirrors were designed to provide early warning of approaching enemy aircraft by detecting the sound of their engine over long distances. The first mirror at Denge was built in 1928 as one of a pair of mirrors to bracket the Acoustic Research Station at Hythe, the second mirror being located at Abbot's Cliff. This was a vertically-mounted reinforced concrete slab with a central shallow dish 20ft (6.1m) in diameter and a frontal plinth mounting for a microphone stand. In 1930 a mirror with a larger, deeper curved bowl of 30ft (9.1m) diameter was installed at the site and also in 1930 a 200ft (61m) strip mirror- a curved, vertical wall- was constructed. Microphones were attached to the curved surfaces and in favourable conditions could pick up aircraft sound 8-24 miles away. The mirrors were superceded by radar in 1935.

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Greatstone Sound Mirror System. Twenty foot sound mirror, thirty foot sound mirror and two hundred foot sound mirror at Denge. The three concrete sound mirrors were built as part of an experimental system to detect aircraft by amplified sound. The mirrors formed part of a test system controlled from the research station at The Roughs, Hythe (TR 13 SW 19). The 'twenty foot' sound mirror was built in 1928. It was still standing in 1994 although the foundations have been undermined by nearby gravel workings. The 'thirty foot' mirror was built in 1929/30. It is still standing, the metal pillar provided for the microphones can still be seen. The 'two hundred foot mirror', built in 1930, is a curved wall with a diameter of 70m. The concrete wall has buttresses behind, with a flat concrete apron in front. The foundations of the south part have been washed away. All of the mirrors are on private land and surrounded by water as a result of gravel working nearby. (1-2)

A photogrammetric survey was undertaken by English Heritage's Metric Survey team in August 2005. The detailed recording has allowed a computer-generated animation of the site to be created. (3)

The site of three sound mirrors at located at Denge (TR 073 218). Sound mirrors were designed to provide early warning of approaching enemy aircraft by detecting the sound of their engine over long distances. The first mirror at Denge was built in 1928 as one of a pair of mirrors to bracket the Acoustic Research Station at Hythe, the second mirror being located at Abbot's Cliff. This was a vertically-mounted reinforced concrete slab with a central shallow dish 20ft (6.1m) in diameter and a frontal plinth mounting for a microphone stand. In 1930 a mirror with a larger, deeper curved bowl of 30ft (9.1m) diameter was installed at the site and also in 1930 a 200ft (61m) strip mirror- a curved, vertical wall- was constructed. (5)

SOURCE TEXT

( 1) General reference

DOE Inspectorate of Ancient Monuments Record Form 04/12/1978

(2) Richard N Scarth 1995 Mirrors by the sea : an account of the Hythe Sound Mirror System based on contemporary letters and reports

Page(s)5, 8, 10, 37

(3) Kent's Defence Heritage

Gazetteer Part 1: KD154

(4) General reference

Research News: Newsletter of the English Heritage Research Department, 3, Spring 2006, 38-41

(5) C S Dobinson 2000 Twentieth century fortifications in England, volume 7. Acoustics and radar: England's early warning systems 1915-1945

Page(s)8-19,156

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ACOUSTICAL RESEARCH STATION
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The site of an Acoustical Research Station built by the Air Ministry in 1922 to develop methods of detecting aircraft using amplified sound, building ...

MONUMENT NO. 1413672
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A sound mirror at Abbot's Cliff built in 1928. Sound mirrors were designed to provide early warning of approaching enemy aircraft by detecting the sou...

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