The sound mirror was part of a chain of similar acoustic devices located on the north east coast extending from the Humber to the Tyne. They were erected to provide early warning of potential attacks on the Acoustic industrial complexes in the north east from ships and Zeppelins during World War I.
Based on an experimental sixteen-foot reflector cut into a chalk cliff near Maidstone in July 1915, the concrete acoustic mirror was a concave segment of a sphere with a trumpet-shaped sound-collector pivoted at the focal point. Listeners used rubber tubes, like a stethoscope, to pick up the noise of approaching engines across the sea, and panned the collector across the mirror to locate the direction. A range of up to twenty miles was claimed for this system, giving several minutes’ advantage over optical or aural observations. The system became less effective as aircraft speeds increased during the 1920s and was superseded by the development of radar from 1932 onwards.
The Kilnsea acoustic mirror was built at some point during the WWI . It was a concrete built structure comprising a thick slab wall, 5.2m long, 2.5m deep and approximately 4m high tapering in thickness to the top with an inclined face on the seaward side. It had a half hexagon shaped profile and unlike the other examples in the north east did not have supporting side walls. On the inclined face there was a shallow concave bowl, approximately 4.5m in diameter and just over a metre deep which acted as the sound reflector. The surface of the bowl was smooth, whereas its rim and the rest of the wall surface has a rough surface `pecked’ with a pick. Approximately 1.5m to the east of the mirror wall was the `sounding column’, a concrete plinth which supported a pyramidal column cast around a metal pipe which extends 1.5m high. This originally supported a swivelling, trumpet-shaped `collector head’, which via two wires and a stethoscope head set, fed detected sounds to a listener who was located nearby, usually in a below ground bunker. A 1980s survey noted a possible entrance to an infilled trench or bunker south of the concrete plinth which would have housed the Listener’s Post.
Featured photo by Mike Higginbottom Interesting Times