Anyone ever heard of this?
Prior to World War II and the invention of radar, acoustic mirrors were built as early warning devices around the coasts of Great Britain, with the aim of detecting airborne invasions. The most famous of these devices still stand at Denge on the Dungeness peninsula and at Hythe in Kent. Other examples exist in other parts of Britain (including Sunderland, Redcar, Boulby, Kilnsea) and Selsey Bill, and Baħar iċ-Ċagħaq in Malta. The Maltese sound mirror is known locally as “the ear” (il-Widna) and appears to be the only sound mirror built outside Great Britain.