Fan Bay Sound Mirrors

No. 1 Sound Mirror, Fan Bay

No. 1 Sound Mirror, Fan Bay


26 October 2016


Fan Bay, Dover, Kent
TR 35221 42799; 51.13611°N, 1.36095°E


Concrete sound mirrors were developed in Britain to provide early warning of approaching aircraft. The dish-shaped acoustic reflectors focused sound waves to a point in front of the mirror where a microphone would be placed, or an operator equipped with a stethoscope would be positioned. The technology became obsolete in the 1930s with the advent of radar.

The two 4.5 metre World War I sound mirrors at Fan Bay, the site of which had been filled in in the 1970s, were excavated by the National Trust as part of its restoration of Fan Bay Deep Shelter. An archaeological dig took place in 2014 to find the mirrors and 600 tonnes of spoil was removed to uncover the devices. The guided tour of Fan Bay Deep Shelter includes an opportunity to view the sound mirrors at close quarters.

Further Reading

Sound Mirrors;
Fan Bay Deep Shelter;
First World War ‘sound mirrors’ restored on White Cliffs of Dover (Centenary News)


Abbot’s Cliff Acoustic Mirror

Acoustic Mirror, Abbot's Cliff

Acoustic Mirror, Abbot’s Cliff


10 February 2013

Abbot’s Cliff, Folkestone, Kent

TR 27188 38605; 51.10170°N, 1.24369°E


Before the advent of radar, there was an experimental programme during the 1920s and 30s in which a number of concrete sound reflectors, in a variety of shapes, were built at coastal locations in order to provide early warning of approaching enemy aircraft. A microphone, placed at a focal point, was used to detect the sound waves arriving at and concentrated by the acoustic mirror. These concrete structures were in fixed positions and were spherical, rather than paraboloidal, reflectors. This meant that direction finding could be achieved by altering the position of the microphone rather than moving the mirror.

The acoustic mirror at Abbot’s Cliff is located on the cliff top almost midway between Folkestone and Dover.

Acoustic mirror (Wikipedia);
Abbot’s Cliff sound mirror (Andrew Grantham)