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Monument details

HER Number:TR 02 SE 12
Type of record:Monument
Name:Acoustic Sound Mirrors at Greatstone-on-Sea, Lydd

Summary

The remains of three large concrete structures, formerly an anti-aircraft Acoustic Detection installation. Microphones were attached to the three reinforced concrete structures in order to pick up the sound of approaching enemy aircraft. The smallest 'sound mirror' was found to be fairly ineffective so it was superseded by a larger dish, 12 m in diameter. This in turn was replaced by a 70m long 'sound wall'. The structures were built by the RAF between 1930-4. They were rendered obsolete by the introduction of radar in 1935, and by advances in aerial technology.

Summary from record 462809:

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.

Images

Sound mirrors at Lydd   © Kent County CouncilSound mirror at Lydd   © Kent County CouncilSound mirror at Lydd showing microphone   © Kent County CouncilSound mirror at Lydd from behind   © Kent County CouncilSound mirror at Lydd   © Kent County CouncilSound mirror at Lydd from behind   © Kent County Council
Grid Reference:TR 0755 2154
Map Sheet:TR02SE
Parish:LYDD, SHEPWAY, KENT

Monument Types

  • SOUND LOCATOR EMPLACEMENT (Modern - 1901 AD to 2000 AD)
  • SOUND MIRROR (Abandoned late 1930s, Modern - 1930 AD to 1938 AD)
  • ANTI AIRCRAFT GUN EMPLACEMENT (Modern - 1939 AD to 1945 AD)
Protected Status:Scheduled Monument 1005119: Listening devices, Greatstone

Full description

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TR 075215. Anti-Aircraft Acoustic-Detection Installation. Three mirror-like bowls of re-inforced concrete to which microphones were attached were designed and built by the RAF 1930-4. The concentrated sound of aircraft could be picked up at distances up to 24 miles under favourable conditions. The introduction of radar rendered them obsolesent by 1935. The southern device is a curved wall 70.0m in diameter buttresses behind and with a flat concrete apron in front. The foundations of the S part have been washed away. The centre device is a shallow saucer buttressed at the back. It has tilted into the gravel pit. The northern device is a hemispherical saucer about 12.0m in diameter with the remains of iron antennae in the centre. It is in a relatively stable condition. (1)

This group of three concrete sound mirrors is near the edge of a flooded gravel quarry. It formed part of a partly experimental system of mirrors along the Kent coast to Dover to give early-warning of the approach of enemy aircraft by detecting the sound of their engines at long range.

The mirrors were built sequentially over a period of three years from 1927: a 20 ft. slab mirror in 1927-8, a 30 ft. bowl version in 1929 and a 200 ft. strip mirror in 1930.

In the case of the 20-ft and 30ft mirrors, sound location was achieved by focussing and collecting the sounds of an aircraft engine striking the concrete mirror in a metal trumpet connected to a stethoscope worn by a listener. For the 20-ft mirror the listener stood at its front and for the 30-ft. version sat in a small chamber below the front of the bowl. By moving the trumpet and recording the angle of best reception of sound the direction to the target could be established. By then matching this to the angle of sound taken from another of the coastal mirrors, it was possible to establish the position and height of the target aircraft and track it in flight. The strip mirror used a row of microphones along its front. The microphone with the best reception indicated the angle to the target.

By means of telephone communication, fighter interceptors could be ordered airborne. The system was tested with varying degrees of success during a number of air defence exercises soon after construction and in to the mid-1930s.

Fixed sound location ceased with the establishment of radar stations in the later 1930s.

Gravel quarrying was undertaken in areas adjacent to the sound mirrors in the post-Second World War period.

The 20 ft. mirror is a monolithic, free-standing block of concrete containing a shallow concave ‘dish’ 20 ft in diameter. The concrete pillar, which supported the listener’s platform, has vanished.

The 30-ft. mirror consists of a reinforced concrete bowl angled skywards and supported at the back and sides with vertical concrete webs. Beneath the bowl and reached down steps is a small chamber for the listener. The fixture for the sound trumpet is in place.

The 200-ft. strip is slightly curved in plan, with a concave section, braced to the rear by a succession of triangular buttresses and fronted by a wide sloping apron. The microphones were originally mounted at the foot of the latter. The debris of a collapsed listening room is on the ground behind the mirror.

Importance
15

The mirrors express a distinct stage of technical innovation in aircraft detection during the pre-radar era. This is a unique collection of the 3 main types of sound mirror used.

Historic importance - A

Structural survival - B

Condition
16

The 20 ft. mirror and the front and south edging of the strip mirror are gradually subsiding into the water-filled gravel quarry. These nationally important structures are at risk from further subsidence.

Current condition - E

Ownership (or tenancy)
17
Hall Aggregates (South Eastern) Ltd.

Use(s) of the site
18

Abandoned.

Statutory protection
19

Scheduled Ancient Monument.

Nature conservation interests
20

SSSI: Dungeness

Public access to the site
21

The mirrors are on private, commercial land with no public right of access but may be viewed over the fence defining the southern boundary of the flooded gravel quarries.

Relationship to other sites
22
The grouping of mirrors was the southern end of a sequence of mirrors along the Kent coast to Dover. Other examples survive at Hythe and at Abbots Cliff.

Potential as part of an economic regeneration package
23

None

Interpretational potential as an educational resource
24
The mirrors would have potential if they could be made publicly accessible.

Tourism potential
25
Likewise

Potential for other beneficial reuse
26
None

Constraints
27
On site heritage presentation is inhibited by the location of the mirrors within a commercially active and restricted area.(2)

From the National Heritage List for England:

Details:

The monument includes three acoustic early warning devices surviving as upstanding and below-ground remains. It is situated on a man-made island surrounded by a lake formed by gravel extraction on the Kent coastline at Greatstone-on-Sea. The acoustic devices form a row from north to south and are positioned to pick up aircraft noise from the east, across the Strait of Dover. The southernmost acoustic early warning device is a sound wall formed by a vertical concave reinforced concrete about 61m in diameter and 7.9m high. It is fronted by a near horizontal, gently sloping, concrete apron. The sound wall is supported by a succession of triangular concrete buttresses to the rear. Microphones were originally mounted at the foot of the horizontal apron, picking up sound reflected off the wall. A listening room was also situated to the rear of the sound wall but no longer survives. The central acoustic early warning device is a sound mirror formed of a circular concave concrete 'dish' about 6.1m in diameter. It is supported on a freestanding block of concrete. The concrete pillar which originally supported the listener's platform is no longer present. The northernmost acoustic early warning device is another sound mirror formed by a hemispherical concrete 'bowl' or 'dish' about 9.1m in diameter. It is angled skywards and supported at the back and sides with vertical reinforced concrete webs. A listening chamber is situated beneath the mirror and reached by steps. The iron antenna for the sound trumpet is still in place, at the front, in the centre of the mirror. The three acoustic early warning devices at Greatstone-on-Sea were built following the establishment of the Acoustical Research Station at the Roughs, Hythe by the Air Ministry in 1922. They were built as part of an experimental system to detect aircraft by amplified sound. The grouping was the southern end of a sequence of mirrors along the Kent coast to Dover. Other examples survive at Hythe and at Abbots Cliff. The three acoustic devices were built sequentially over a period of about four years. In 1927-8 the smallest sound mirror was built and in 1929 the larger hemispherical mirror was constructed. Finally, the sound wall was added in 1930. The devices worked by focussing and collecting the sound waves of an aircraft engine striking the concrete mirror in a metal trumpet connected to a stethoscope worn by a listener. For the smaller mirror the listener was positioned at the front but for the hemispherical mirror a listening chamber was provided below. By moving the trumpet and recording the angle of best reception of sound the direction to the target aircraft could be established. This was then matched with the sound taken from another coastal sound mirror to establish the position and height of the target aircraft and track it in flight. The sound wall used a number of microphones at the front of the wall; the microphone with the best reception indicating the angle to the target. Following the detection of an enemy aircraft, fighter interceptors could be ordered airborne to counter the threat. The system was tested with varying degrees of success during a number of air defence exercises in the early 1930s, until it was rendered obselete by the advent of radar. In favourable conditions the acoustic devices could pick up aircraft sound between 8 and 24 miles away.

Reasons for Designation:

The use of aircraft as offensive weapons was a significant 20th century development in the history of warfare, and provoked new systems of strategic air defence. Experiments in early warning systems started before 1920 with the new possibility of attacks by airships. Early warning was initially based on visual spotting, but acoustic detection devices were soon developed. The principle of acoustic detection is relatively straightforward: an acoustic receiving dish reflected the sound of distant aircraft engines onto a focal point where it was detected by a listener or, later, by microphones. There were three main types of acoustic device: mirror, wall and disc. Mirrors were upright concave bowls between 3m and 4m in diameter, usually contained in concrete slab walls; the walls were curved vertical structures up to 61m in length; the disc system used horizontally-set concave bowls designed for use in pairs as aircraft passed overhead to measure speed. At their most sophisticated, the devices could identify the sounds of surface vessels or aircraft up to 25 miles (c40km) away. Research into acoustic early warning was carried out in a number of countries during the early 20th century. British experiments at the Royal Flying Corps research establishment at Farnborough tested parabolic acoustic sound reflectors of varying shapes and curvature, and led to the first true sound mirror at Binbury Manor in the summer of 1915, a circular disc cut directly into a low chalk cliff. The first operational acoustic reflectors were a pair of adjustable mirrors erected on the Kent coast in 1917, followed by a series of concrete static mirrors established on the north east coast later in World War One. Further experiments were carried out after the war. This led to the building of a complex chain of mirrors on the Kent coast around Hythe in the late 1920s. Unrealised plans were also drawn up for an ambitious scheme to be installed around the Thames estuary. Acoustic devices always remained susceptible to interference from extraneous noises and adverse weather. As aircraft performance increased, the time between detection and arrival of enemy aircraft rapidly shortened and reduced the value of acoustic devices as an early warning system. By 1936 the technology of radar had replaced acoustic methods as the main form of early warning, although acoustic systems remained in use at anti-aircraft and searchlight batteries, and as backup systems in the event of radar being jammed. A national survey of acoustic early warning devices has identified only around 11 sites where remains of acoustic detection survive. Field evidence of this important aspect of the 20th century defence of Britain is thus rare and all surviving examples are considered to be of national importance. Despite some damage in the past, the three acoustic early warning devices at Greatstone-on-Sea are well preserved. They are of considerable historic interest as one of the earliest forms of airborne early warning system, following the development of aerial warfare in the early 20th century. The devices express a distinct stage of technical innovation in aircraft detection, which preceded the development of radar. They also have group value as a unique collection of the three main types of acoustic devices used in the early 20th century.

Description from record 462809:
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).


<1> OS Card / NAR index entry, DOE (IAM) Rec Form 4.12.78 (Unpublished document). SKE6461.


<1> VIRTUAL CATALOGUE ENTRY TO SUPPORT NAR MIGRATION (Unspecified Type). SWX23962.


<2> Mirrors by the sea : an account of the Hythe Sound Mirror System based on contemporary letters and reports, pp 5, 8, 10, 37 (Unspecified Type). SWX23605.


<2> Victor Smith and Andrew Saunders, 2001, Kent's Defence Heritage (Unpublished document). SKE6956.


<3> Kent's Defence Heritage (Unspecified Type). SWX23591.


<3> Ordnance Survey, 1928-1947, Ordnance Survey 1:2500 4th edition 1928-1947, 1938/1:2500 (Map). Ske12644.


<4> VIRTUAL CATALOGUE ENTRY TO SUPPORT NAR MIGRATION (Unspecified Type). SWX23977.


<4> Ordnance Survey, 2009, OS MasterMap, 2009/MasterMao (Cartographic materials). SWX15710.


<5> Twentieth century fortifications in England, volume 7. Acoustics and radar: England's early warning systems 1915-1945, pp 8-19,156 (Unspecified Type). SWX23709.


<6> Vertical aerial photograph reference number (Unspecified Type). SWX23761.


<7> 1946, NMR CPE/UK/1752 3002-3 21-SEPT-1946 (Photograph). SWX23759.


<8> Oblique aerial photograph reference number (Unspecified Type). SWX23619.


<9> Oblique aerial photograph reference number (Unspecified Type). SWX23620.


<10> Ordnance Survey Map (Scale / Date) (Unspecified Type). SWX23649.

Sources and further reading

Cross-ref. Source description
<1>Unpublished document: OS Card / NAR index entry. DOE (IAM) Rec Form 4.12.78.
<1>(No record type): VIRTUAL CATALOGUE ENTRY TO SUPPORT NAR MIGRATION.
<2>Unpublished document: Victor Smith and Andrew Saunders. 2001. Kent's Defence Heritage.
<2>(No record type): Mirrors by the sea : an account of the Hythe Sound Mirror System based on contemporary letters and reports. pp 5, 8, 10, 37.
<3>Map: Ordnance Survey. 1928-1947. Ordnance Survey 1:2500 4th edition 1928-1947. 1938/1:2500.
<3>(No record type): Kent's Defence Heritage.
<4>Cartographic materials: Ordnance Survey. 2009. OS MasterMap. 2009/MasterMao.
<4>(No record type): VIRTUAL CATALOGUE ENTRY TO SUPPORT NAR MIGRATION.
<5>(No record type): Twentieth century fortifications in England, volume 7. Acoustics and radar: England's early warning systems 1915-1945. pp 8-19,156.
<6>(No record type): Vertical aerial photograph reference number.
<7>Photograph: 1946. NMR CPE/UK/1752 3002-3 21-SEPT-1946.
<8>(No record type): Oblique aerial photograph reference number.
<9>(No record type): Oblique aerial photograph reference number.
<10>(No record type): Ordnance Survey Map (Scale / Date).

Related records

TR 23 NE 32Parent of: Abbot's Cliff Sound Mirror (Building)
MWX51471Parent of: Second World War anti invasion scaffolding, Greatstone-on-Sea (Monument)
MWX51458Parent of: Second World War bomb crater (Monument)

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