From The Neolithic To The Sea: A Journey From The Past To The Present

Kilnsea Sound Mirror

Sound Mirror
53° 37' 38.4" N 0° 7' 54.0" E

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Kilnsea Sound Mirror is located near Kilnsea, Yorkshire.It is the only listed ‘building’ in Kilnsea.

In World War I the Germans used Zeppelins to make bombing raids on eastern England. In order to have some warning of their approach, acoustic sound mirrors made of concrete were erected at various points along the coast. The Kilnsea sound mirror was probably built in 1916/17. It is in the shape of a half-hexagon, with an inner concave circular disc to amplify sound. It is 16 feet high and 17 feet wide. In front of the concrete disc is a plinth, with a pipe on which would have been mounted a trumpet-shaped 'collector head' which was a basic microphone.

In May 1915 Zeppelin and Shutte-Lanz airships of the German Army and Navy started bombing targets around the Humber and Thames estuaries. By 1917 the airships were being replaced by twin engined Gotha and Giant aeroplanes. In total 300 tons of bombs were dropped on Britain during the First World War causing some 5,000 casualties, a third of which were fatalities. Some form of early warning system was needed, especially to counter the night raids.

Following encouraging experiments with a four foot diameter prototype built by a Professor Mather a 16' mirror was cut into a chalk cliff face at Binbury Manor between Sittingbourne and Maidstone in July 1915. The mirror was shaped to form part of a sphere and a sound collector was mounted on a pivot at the focal point. The collector was usually a trumpet shaped cone connected to the ears of the listener with rubber tubes but experiments with microphones were under way before the end of the war. The listener would move the sound collector across the face of the mirror until he found the point where the sound was loudest. Bearings to the target could then be read from vertical and horizontal scales on the collector.

The reflectors are in fact hemispherical mirrors. Able to detect range over twenty statute miles on a good day, they could also detect direction. Acoustic mirrors had a limited effectiveness, and the increasing speed of aircraft in the 1930s meant that they would already be too close to deal with by the time they had been detected. The development of radar put an end to further experimentation with the technique. Nevertheless, there were long-lasting benefits. The acoustic mirror programme, led by Dr William Sansome Tucker, had given Britain the methodology to use interconnected stations to pin point the position of an enemy in the sky. The system they developed for linking the ranging stations and plotting aircraft movements was given to the early radar team and contributed to their success in WW2.