The acoustic mirrors were built on the south and northeast coasts of England between about 1916 and the 1930s. Popularly known as 'listening ears', they were intended to provide early warning of incoming enemy aeroplanes and airships about to attack coastal towns.

With the development of faster aircraft the mirrors became less useful, as a plane would already be within sight by the time it had been located. They were finally rendered obsolete by the advent of radar in the mid-late 1930's.

A very good article about the mirrors can be found here.

Writer Paul Morris in front of one of the three mirrors at Denge Marsh.