Largest acoustic mirror

Acoustic mirrors were developed by the British as an experimental early warning system allowing the detection of incoming enemy aircraft. The largest examples were built in the 1920s and early 1930s near Dungeness, UK and Maghtab, Malta. These two were built using the same design and consisted of a curved concrete wall 200 feet long by 27 feet high which focused sound into a "listening trench" in which microphones were installed. Acoustic mirrors as an early warning system became obsolete with the invention of radar.