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Monument details

HER Number:TR 34 SE 412
Type of record:Monument
Name:Fan Bay Sound Mirrors

Summary

Two bowl mirrors dating from 1917 and the 1920s designed to give early-warning of approaching enemy aircraft by detecting the sound of their engines. The mirrors formed part of a chain of experimental sound mirrors at points along the Kentish coast.


Grid Reference:TR 3521 4279
Map Sheet:TR34SE
Parish:DOVER, DOVER, KENT

Monument Types

Protected Status:Scheduled Monument 1442235: A pair of acoustic mirrors at Fan Bay

Full description

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An anti-aircraft acoustic-detection device. Two mirrors exist on the site. They are examples of a small number of surviving sound mirrors nationally including nearby Greatstone (TR 02 SE 12) and at Abbotscliff (TR 23 NE 32).

One mirror dates from 1917 and is twenty feet in diameter, the other being thirty feet in diameter and dates from the 1920s. They were intended to give early-warning of approaching enemy aircraft by detecting the sound of their engines at long distance and formed part of a chain of partly experimental sound mirrors at points along the Kentish coast. The earlier mirror lies at a shallower angle (two degrees) than the later dish and it appears it was abandoned in favour of the larger mirror which sits at an eight degree angle.

Both are supported at the back and at either side by buttresses of steel girders fixed into concrete ground pedestals and by two intervening webs of concrete, with metal supports and concrete bearers at the front of the bowl. Spalling shows the bowl to have a formwork of steel mesh into which a mortar of sand and cement was plastered. In the lower part of the front of the bowl is the cut-off pillar for the sound trumpet.

In 2017 the mirrors were designated as a Scheduled Monument. From the National Heritage List for England:

List entry Description

Summary of Monument

Pair of acoustic mirrors, the eastern dating from circa 1916 and the western from 1920-23. Constructed during the Second World War, the remains of the toilet blocks in front of the mirrors, and the tunnel entrances to the deep shelter behind, are not included in the scheduling. .

Reasons for Designation

The pair of sound mirrors at Fan Bay, near Dover, is scheduled for the following principal reasons: * Potential: this site has the potential to enhance our understanding of the development of early warning systems; * Survival: both mirrors survive extremely well, largely due to the fact that they have been covered for circa 40 years; * Rarity: the c1916 mirror is one of the earliest surviving examples, and it is highly unlikely that further sites remain to be discovered nationally; * Historic importance: excluding the later sites at Denge and Hythe, all other designated examples consist of a single surviving freestanding mirror. The fact that there are two mirrors at Fan Bay further enhances their importance and demonstrates the evolution of this early warning system during the First World War and interwar period; * Documentation: the importance of this site in a national context is underpinned by our previous designation work at other related sites. .

History

Systems of acoustic detection of in-coming aircraft were developed from just before the First World War, and grew out of established techniques for locating enemy artillery positions. Early sites were based on acoustic detection and many of the rare surviving sound mirrors along the south and east coasts are scheduled and/or listed.

The use of aircraft as offensive weapons in the C20 was a significant development in the history of warfare, and provoked new systems of strategic air defence. Experiments with early warning systems began during the First World War with the possibility of attack by airships and aircraft. Early warning was initially based on visual spotting, but acoustic detection devices were soon developed. The principle of acoustic detection is relatively straightforward: a 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; the walls were curved vertical structures up to 61m in length; the disc system used horizontal concave bowls designed for use in pairs as aircraft passed overhead to measure speed. At their most sophisticated, these devices could identify the sounds of surface vessels or aircraft up to 25 miles away.

Experiments at the Royal Flying Corps research establishment at Farnborough tested parabolic sound reflectors of varying shapes and curvature, and led to the first true sound mirror at Binbury Manor in July 1915, a circular disc cut directly into a low chalk cliff. Similar installations were subsequently installed during the First World War along the Sussex, Kent and north-east coasts. Further experiments continued after the war, leading to a complex chain of mirrors on the Kent coast around Hythe in the late 1920s. 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. The development of Radar by 1936 essentially rendered the mirrors obsolete, but many were retained for use as a backup when Radar systems were jammed, or to complement searchlights.

It is probable that the eastern mirror at Fan Bay was constructed soon after the successful testing of the mirror at Binbury Manor, given the proximity of the site and the similarities in construction methods. It was designed and installed by Major Mather and Lieutenant Rogers of the Royal Engineers, Dover Anti-Aircraft Defences. Interestingly, the Major Mather referenced may be the same Professor Mather who was the designer of the mirror at Binbury Manor. It was commonplace for academics to be called into the armed services and given military rank. Fan Bay also commands a vital position overlooking the English Channel and is a logical place to put an early warning device.

Although an exact date of construction cannot be ascertained, this mirror is documented as being active during a raid on 1-2 October 1917, when the mirror was able to detect the sound of enemy aircraft flying down the Channel at a distance of 12-15 miles (Scarth, 1999, p22). This makes it one of the earliest surviving examples of an acoustic mirror. The western mirror is estimated to date between 1920 and 1923. Fuller discussion of the measurements follows in the details section, but both mirrors at Fan Bay are approximately 15ft in diameter. Larger 20ft mirrors appear to have become the standard size for new mirrors in circa 1923.

All surviving examples dating from the First World War are singular examples of acoustic mirrors. The later sites at Hythe and Denge consist of two and three mirrors respectively, but Fan Bay is the only example of a First World War mirror being complemented by another. Also, the Fan Bay mirrors appear to be unusual surviving examples of such a construction method, with all other surviving acoustic mirrors freestanding and constructed of concrete. The 1915 example documented at Binbury Manor was built in a similar way, but is no longer extant as of 2017. It has been suggested that the chalk mirrors were pioneering structures, preceding the construction of concrete mirrors (Scarth, 1999, p18). This is likely the case with the eastern mirror; however, the western mirror was likely constructed later but in a similar fashion due to the topography of the site.

During the Second World War, a gun battery was constructed at Fan Bay, along with an associated deep shelter, providing a safe haven for the gunners stationed on the cliff top. Two exit tunnels link the sound mirrors to the deep shelter behind. Directly in front of the mirrors at the exit of these tunnels, two lavatory buildings were constructed. The sound mirrors and all entrances to the deep shelter were covered over in the 1970s, as part of an effort to remove any evidence of obsolete military installations in Kent. They remained in this state until the land was purchased by the National Trust in 2012. The mirrors and deep shelter were subsequently excavated in 2014 and are now open to the public as a visitor attraction, as of 2017. .

Details

PRINCIPAL ELEMENTS The monument includes two acoustic mirrors at Fan Bay, Dover, located at National Grid Reference TR 35219 42790. The eastern mirror was constructed first in circa 1916, part way down the cliff side, with the western mirror following circa 1920-23. Fan Hole is a deep depression at the top of the White Cliffs between Dover and St Margaret’s at Cliffe, overlooking Fan Bay. The monument has an uninterrupted view over the English Channel, essential for the function of the mirrors.

DESCRIPTION Both mirrors face out towards the sea and are carved directly into the side of the chalk cliff, faced in a layer of concrete approximately 10cm thick. The eastern mirror is set at a 16 degree angle, facing slightly toward the sky. It has an overall diameter of approximately 4.5m, whilst the spherical surface has an approximate radius of approximately 3m. There are no side walls, a feature which is present on a number of surviving comparative examples. The western mirror also has approximately an 4.5m diameter but there are some key differences. The spherical surface is larger, with a radius of approximately 5.1m, giving a larger focal length and consequently a longer range of sound detection. This mirror is also essentially vertical and has a triangular concrete apron above, which would have presumably been added to combat drainage issues. This addition is representative of this period of construction.

Neither mirror retains any evidence of the listening gear or mounting. A line drawing in a 1932 War Office publication shows the listening gear at one of the Fan Bay mirrors, which is mounted on a vertical tube protruding from a small concrete pillar just above ground level in front of the mirror. No evidence of this structure was seen and it is presumed that the concrete pillars were destroyed when the lavatory buildings were constructed in 1940-41.

EXCLUSIONS All modern paths and fences, the remains of the toilet blocks erected in the Second World War to the front of the mirrors, and the entrances to the deep shelter behind are excluded from the scheduling. (3)


Sound location was achieved by collecting and focussing the sounds of an aircraft engine striking the concrete ‘mirror’ in a metal trumpet connected to a stethoscope worn by a listener who 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 of the target could be established. By matching this to an 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. Fighter interceptors could then be ordered airborne. This system was tested with varying degrees of success during a number of air defence exercises in the early-mid 1930s but was abandoned with the establishment of RADAR stations in the later 1930s.

The mirrors have recently (2012) been excavated during work at the nearby Fan Bay Deep Shelter and are currently (2016) in the process of being conserved by the national Trust.

GIS depiction corrected following communication with John Guy. (1)

Text Below taken from Dover Archaeological Group (January, 2015), Source (2).

The North Sound mirror (dated to 1917) has been measured at 4.66m (15 3.5 inch), this would provisionally suggest that the sound mirror is formed from a sphere whose diameter is 5.75m (almost 19 feet). The dish was supported by a smooth concrete surround and the whole structure is built at an angle of 16 degrees from the vertical, presumably pointing to an area of the sky from which incoming aircraft were more likely to approach.

There was some damage to the northern edge of this sound mirror caused during its burial in the 1970's. There is also a deeper crack in the fabric, which appears to originate from the damage in the top north corner, which may be the result of root penetration as it does not appear in the 1970's photographs.

Situated 1.20 metres in front of the Northern side of the north sound mirror and extending northwards acorss the front of the entrance of the North tunnel of the deep shelter is a substantial brick-built blast wall. This was constructed in 1940-41 to guard the tunnel against any potential explosions. The blast wall stands to a height of 2.49 metres (8ft 2in) and is built of courses of red and yellow frogged bricks. The wall begins at 0.59m thick from its base up to a height of 1.23m at which point it narrows on both sides to 0.36m. The wall is currently being supported by soil in front of it, against which it is leaning.

The remains of a one-inch steel water pipe were discovered during the excavation by the National trust, part of which remains wedged into and along the narrower brickwork. A singluar circle steel fixing bracket marks the original location of the water pipe and remains in situe at a heigh 1.09m above the ground surface. This area is shown in the 1940's plan of the site as an 'ablutions bench.'

At a distance of 0.76m south of the blast wall is a brick-built latrine block also dating to 1940/41. This latrine structure was constructed of the same red and yellow frogged bricks as those making up the blast wall. They have been identified as belonging to Smeed Dean of Sittingbourne and another manufacturer with the letters EI who has yet to be identified. A dividing wall was revealed, one brick in thickness, between the two cubicles of the North latrine Block. This latrine is believed to have been for the use of officers only and measure 0.89m (2ft 11in) wide and 1.61m (5ft 3in) long. The southern wall of this structure appears to stand to its full original height at 2.03m (6ft 8in) althought its concrete roof does not survive.

The depth of the south mirror, which is believed to have been constructed around the 1920's, has been measured as being formed from a sphere whose diameter is roughly 10.87m or 35ft 4inches. This shallower dish would enable the South Mirror to detage incoming aircraft at a much longer range than its predecessor. The mirror ws sumounted by a triangular-shaped concrete apron, believed to have been designed to prevent rainwater from seeping behind the sound mirror.

The blast wall constructed in front of the South Sound mirror is 1.20m from its front edge. It stands at 2.60 (8ft 6in) high and is similarly built of red and yellow frogged bricks. The wall is 0.60m thick from its base to a height of 1.42m at which it narrows, only on its outer side, to a thickness of 0.36m. Unlike the Northern wall there is no evidence of a water supply, although at a height of 1.46m above the ground surface there is a series of three horizontal 0.10m wide slots in the mortar between two courses of bricks.

Similarly to the Northern sound mirror there is a 1940's latrine block in front of the South Sound Mirror. Within this latrine there were four cubicles, seperated by single brick partitions. As this latrine is believed to have been used by the 'other ranks' they are smaller than the Northern latrine. They measure 0.78m (2ft 7in) wide and 1.30m (4ft 3in) long. Unlike at the northern latrine, parts of the concrete roof of the structure still remain in situ, indicating that the buidling stood to a height of 1.88m (6ft 2in). (2)


<1> John A. Guy, 2013, Verbal communication from John Guy, defence expert working in the Dover area (Verbal communication). SKE24831.

<2> Dover Archaeological Group, 2015, Fan Hole deep Shelter and Sound Mirrors, St Margaret's at Cliffe, Dover. Watching Brief (Unpublished document). SKE31707.

<3> Historic England, National Heritage List for England (Index). SKE29372.

Sources and further reading

Cross-ref. Source description
<1>Verbal communication: John A. Guy. 2013. Verbal communication from John Guy, defence expert working in the Dover area.
<2>Unpublished document: Dover Archaeological Group. 2015. Fan Hole deep Shelter and Sound Mirrors, St Margaret's at Cliffe, Dover. Watching Brief.
<3>Index: Historic England. National Heritage List for England.