The business end of the Sound Mirror, as compared with the earlier view. Completed in 1928 at a cost of £650, this 20-foot diameter example at Abbots Cliff (Lyddon Spout) is one of only two built. The concept was that sound from approaching aircraft engines would be reflected by the mirror to a microphone placed in front of the dish (the microphone is missing here). Developed from primitive sound mirrors used during WWI, concrete sound mirrors were further developed, via the 30-foot diameter version and culminating in 1930 with a 200-foot long, 26-foot high wall, the sole example of which can be seen at Denge, near Dungeness. This wall was able to detect aircraft at a range of up to 30 miles at a time when the next best detection system was the human ear, which could only detect them at 6.5 miles distance. A mix of mirrors and the wall were trialled during the early '30s when it was found that central control of detection was essential. By 1935, there was a plan to defend the Thames Estuary with two 200-foot walls and eight 30-foot mirrors, but this folded when early radar experiments that year proved much more effective, with even the early trials detecting aircraft at over 40 miles. Perhaps the true legacy of these edifices was the need for the central control of detection and thus maximising the ability to respond to raids, something that was so successfully put into practice during the Battle of Britain in 1940. 

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