Today they’re usually called ‘sound mirrors’ or ‘listening ears’ – and to a certain sort of person they are one of the holy grails of aircraft surveillance.
The ‘big ears’ of Kent built to keep Blighty safe from European invaders, Sophie Campbell, The Telegraph, 17 April 2018.
There will be two open days at the Denge sound mirrors this year. The open days are scheduled for 10:00 to 15:00 on Saturday 7 July 2018 and Saturday 1 September 2018.
There will be a cash-only charge of £5 per adult, £2.50 per child (RSPB members free).
The open days are generally the onkly way for the public to access the Denge listening ears close up, although there have also been some photography days, so it might be worth keeping an eye out, if you are seriously interested.
(Please note that andrewgrantham.co.uk has no connection to the open day, the RSPB or anything else! Please check the details with the RSPB before visiting)
A pair of Richter Spielgeräte concrete sound mirrors in a small park on the banks of the River Thames at Kew in west London. Photographs taken on 11 March 2018.
This tweet about the Sunderland sound mirror was a little bit popular!
Delighted to discover Sunderland's sound mirror hidden behind the VW garage in Fulwell. It was built around 1917 to detect Zeppelins after deadly airship attacks on the city in 1916. More info athttps://t.co/Qn1K2qXZ2R pic.twitter.com/H9Wvbwd48d
— Birmingham 81 (@Birmingham_81) January 22, 2018
Kent, UK. Between the World Wars, before the invention of radar, parabolic sound mirrors were used experimentally as early-warning devices by military air defence forces to detect incoming enemy aircraft by listening for the sound of their engines. #drone #dji pic.twitter.com/ncSkWVMcww
— Bryan Bruner (@Therobotarmy1) January 3, 2018
Acoustic Mirror – sketchbook drawing of an acoustic mirror for a current project, also known as a sound mirror, there’s lots of lovely stuff on them here: https://t.co/8Ro14bdA7z #wip #workinprogress #soundmirrors https://t.co/0xTjf86FoM pic.twitter.com/HYovst5Muw
— Brian McHenry (@lostcont) January 20, 2018
The National Heritage List for England identifies the buildings, sites and landscapes which receive special protection.
— Historic England (@HistoricEngland) December 19, 2017
Accoustic Mirrors, Fan Bay, St Margaret’s at Cliffe, Kent
The threat of aerial warfare in the 20th century provoked new systems of strategic air defence. Acoustic mirrors reflected the sound of distant aircraft onto a focal point where it was detected. The examples at Fan Bay are unusual as they are carved from the cliff face. The eastern mirror is one of the earliest, dating from about 1916. A second mirror dates from the early 1920s. The development of radar in the 1930s rendered sound mirrors obsolete. (Scheduled Monument. List Entry Number: 1442235)
There is a mention of sound mirrors in the article Mirrors and smoke: A. V. Hill, his Brigands, and the science of anti-aircraft gunnery in World War I, William Van der Kloot. DOI: 10.1098/rsnr.2010.0090 Published 20 July 2011.