A pair of acoustic mirrors at Fan Bay


Heritage Category:
Scheduled Monument
List Entry Number:
Date first listed:
Location Description:
Pair of acoustic mirrors, overlooking Fan Bay, Dover. Approximate OS grid reference TR 35218 42790.


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The building or site itself may lie within the boundary of more than one authority.

Location Description:
Pair of acoustic mirrors, overlooking Fan Bay, Dover. Approximate OS grid reference TR 35218 42790.
Dover (District Authority)
St. Margaret's At Cliffe
National Grid Reference:


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.


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.


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.


Books and journals
Cole, C, Cheesman, EF, The Air Defence of Britain 1914-18, (1984)
Saunders, AD, Smith, V, Kent's Defence Heritage, (2001), pp.132-8
Scarth, RN, Echoes from the Sky: A Story of Acoustic Defence, (1999)
A history of Fan Bay Dep Shelter by Kent Archaeology, accessed 02 December 2016 from http://khd.kentarchaeology.org.uk/2014/10/fan-bay-deep-shelter/
Website containing further details and photographs of surviving acoustic mirrors, accessed 02 December 2016 from http://www.andrewgrantham.co.uk/soundmirrors/


This monument is scheduled under the Ancient Monuments and Archaeological Areas Act 1979 as amended as it appears to the Secretary of State to be of national importance. This entry is a copy, the original is held by the Department for Digital, Culture, Media and Sport.

End of official listing

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