Ancient Monuments

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A pair of acoustic mirrors at Fan Bay

A Scheduled Monument in St. Margaret's at Cliffe, Kent

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Latitude: 51.136 / 51°8'9"N

Longitude: 1.3609 / 1°21'39"E

OS Eastings: 635218.368566

OS Northings: 142790.232953

OS Grid: TR352427

Mapcode National: GBR X2V.NX4

Mapcode Global: VHLHC.JPTZ

Entry Name: A pair of acoustic mirrors at Fan Bay

Scheduled Date: 26 May 2017

Source: Historic England

Source ID: 1442235

County: Kent

Civil Parish: St. Margaret's at Cliffe

Traditional County: Kent

Lieutenancy Area (Ceremonial County): Kent


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.

Source: Historic England


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.

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.

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.

Source: Historic England

Reasons for Scheduling

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.

Source: Historic England


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
Website containing further details and photographs of surviving acoustic mirrors, accessed 02 December 2016 from

Source: Historic England

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