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Historic England Research Records

Greatstone Sound Mirror System

Hob Uid: 462809
Location :
Kent
Shepway
Lydd
Grid Ref : TR0753221506
Summary : Three early Twentieth Century sound mirrors are located at Denge, on the Dungeness peninsula. They are of three different types, representing three successive stages of the development of early-warning anti-aircraft technology. The sound mirrors were rendered obsolete by the development of radar by 1935. The three concrete sound mirrors are clearly visible on aerial photographs of 1946, along with several ancillary structures which have since been removed. These features were all mapped from aerial photographs as part of the South East RCZAS NMP project. The sound mirrors themselves are still extant, and are scheduled under the County Number of KE378.
More information : 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. 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. (5)

The three sound mirrors described above, along with several ancillary structures such as two rectangular sheds, are visible on vertical aerial photographs of 1946. The site, at that time enclosed by a fence and rectangular in plan, is centred at TR 0755 2154. It measures approximately 140m east to west and 175m north to south.

The smallest, earliest sound mirror is centred at TR 0755 2159. It takes the form of an upright circular dish, which from the aerial view measures 7m north to south by 2m east to west. The second mirror lies 14m to the north-east, at TR 0756 2161. It is similar in form to the first one, and measures 11m north to south by 6m east to west. The third mirror takes the form of a curved wall, and lies to the south at TR 0752 2150. The wall, together with its forecourt, measures a maximum of 32m east to west and 64m north to south. All three mirrors are orientated just slightly to the north of due east, although the two smaller circular sound mirrors could both be rotated on their bases.

A tramway from Greatstone-on-Sea leads westwards into this site. The tramway is marked on the 1:2500 scale Ordnance Survey map of 1940, so has not been transcribed from aerial photographs. The 1940 Ordnance Survey map records some of the features of the sound mirror site, but not others. It was therefore decided to map from aerial photographs all the features contained within the rectangular perimeter fence so as to provide a coherent interpretation of the whole complex.

A large hollow cylindrical structure is visible on aerial photographs towards the north of the sound mirror enclosure. It is centred at TR 0760 2159, and measures approximately 11m in diameter. It is located on a circular area of hardstanding which measures 17m across. The function of this structure is not certain; it has been suggested that it may be a storage tank, or alternatively a base for some kind of experimental range/direction-finding device associated with the sound mirrors. It also lies just to the south of a line of Second World War anti-invasion scaffolding which passes right through the site between the two smaller sound mirrors. It is therefore also possible that the cylindrical structure is some form of Second World War anti-aircraft gun emplacement. Another branch of the defensive scaffolding also leads into the site further to the south (Monument number 1533928).

Just within the southern perimeter of the site (TR 0758 2146) there is a group of earthworks in a circular arrangement, visible on the aerial photographs of 1946. They may be either the remains of a cylindrical structure like the one just described above; or alternatively the foundations of a further experimental sound mirror that was moved or dismantled. The irregularly shaped outer banks extend across an area measuring approximately 22m east to west, and 19m north to south. The inner bank is a smooth circular ring of 11m in diameter (which is the same as the cylindrical structure to the north). A perfectly circular pit of 3m in diameter lies in the centre of this ring.

By the time of the vertical aerial photograph of 2007, over half of the area of the rectangular enclosure had been excavated for gravel extraction, and is now a lake. The three concrete sound mirrors are still visible on this photograph, although the two circular structures and the rectangular buildings have been removed (the site of the southernmost circular structure is now in the lake) (6-11).

Sources :
Source Number : 1
Source : VIRTUAL CATALOGUE ENTRY TO SUPPORT NAR MIGRATION
Source details : DOE Inspectorate of Ancient Monuments Record Form 04/12/1978
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Source Number : 2
Source : Mirrors by the sea : an account of the Hythe Sound Mirror System based on contemporary letters and reports
Source details :
Page(s) : 5, 8, 10, 37
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Source Number : 11
Source : Vertical aerial photograph reference number
Source details : EARTH.GOOGLE.COM 02-MAY-2007 ACCESSED 28-FEB-2011
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Source Number : 3
Source : Kent's Defence Heritage
Source details : Gazetteer Part 1: KD154
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Source Number : 4
Source : VIRTUAL CATALOGUE ENTRY TO SUPPORT NAR MIGRATION
Source details : Research News: Newsletter of the English Heritage Research Department, 3, Spring 2006, 38-41
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Source Number : 5
Source : Twentieth century fortifications in England, volume 7. Acoustics and radar: England's early warning systems 1915-1945
Source details :
Page(s) : 8-19,156
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Source Number : 6
Source : Vertical aerial photograph reference number
Source details : NMR FNO/25 6049-6050 26-JUN-1942
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Source Number : 7
Source : Vertical aerial photograph reference number
Source details : NMR CPE/UK/1752 3002-3 21-SEPT-1946
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Source Number : 8
Source : Oblique aerial photograph reference number
Source details : NMR TR 0821/3 RAF/CAL/UK/1 98 02-JUN-1947
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Source Number : 9
Source : Oblique aerial photograph reference number
Source details : NMR TR 0822/1 RAF/CAL/UK/1 99 02-JUN-1947
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Source Number : 10
Source : Ordnance Survey Map (Scale / Date)
Source details : 1:2500 Kent 1940
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Monument Types:
Monument Period Name : 20th Century
Display Date : Built between 1928 and 1930
Monument End Date : 1935
Monument Start Date : 1928
Monument Type : Sound Mirror, Sound Locator Emplacement, Building
Evidence : Structure, Documentary Evidence, Building, Demolished Building
Monument Period Name : Second World War
Display Date : Second World War
Monument End Date : 1945
Monument Start Date : 1939
Monument Type : Anti Aircraft Gun Emplacement
Evidence : Structure, Demolished Structure, Earthwork, Destroyed Monument

Components and Objects:
Related Records from other datasets:
External Cross Reference Source : SMR Number (Kent)
External Cross Reference Number : MKE3769
External Cross Reference Notes :
External Cross Reference Source : Scheduled Monument Legacy (County No.)
External Cross Reference Number : KE 378
External Cross Reference Notes :
External Cross Reference Source : NBR Index Number
External Cross Reference Number : 98765
External Cross Reference Notes :
External Cross Reference Source : National Monuments Record Number
External Cross Reference Number : TR 02 SE 12
External Cross Reference Notes :

Related Warden Records :
Associated Monuments : 464039
Relationship type : General association
Associated Monuments : 1413672
Relationship type : General association
Associated Monuments : 1533844
Relationship type : General association
Associated Monuments : 1533845
Relationship type : General association
Associated Monuments : 1533928
Relationship type : General association

Related Activities :
Associated Activities : INVESTIGATION BY RCHME/EH ARCHITECTURAL SURVEY
Activity type : ARCHITECTURAL SURVEY
Start Date : 1995-11-14
End Date : 1995-11-14
Associated Activities : LANDSCAPES OF WAR RECORDING PROJECT
Activity type : MEASURED SURVEY
Start Date : 2008-01-01
End Date : 2009-12-31
Associated Activities : SOUTH EAST RAPID COASTAL ZONE ASSESSMENT SURVEY NMP (COMPONENT 2) GLOUCESTERSHIRE COUNTY COUNCIL
Activity type : AERIAL PHOTOGRAPH INTERPRETATION
Start Date : 2011-01-10
End Date : 2011-07-10