26 October 2016
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.