World War I acoustic mirror 335m north east of Kilnsea Grange


Heritage Category:
Scheduled Monument
List Entry Number:
Date first listed:


Ordnance survey map of World War I acoustic mirror 335m north east of Kilnsea Grange
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The building or site itself may lie within the boundary of more than one authority.

East Riding of Yorkshire (Unitary Authority)
National Grid Reference:
TA 41071 16644

Reasons for Designation

The use of aircraft as offensive weapons was a significant 20th century development in the history of warfare, and provoked new systems of strategic air defence. Experiments in early warning systems started before 1920 with the new possibility of attacks by airships. Early warning was initially based on visual spotting, but acoustic detection devices were soon developed. The principle of acoustic detection is relatively straightforward: an acoustic receiving dish reflected the sound of distant aircraft engines onto a focal point where it was detected by a listener or, later, by microphones. There were three main types of acoustic device: mirror, wall and disc. Mirrors were upright concave bowls between 3m and 4m in diameter, usually contained in concrete slab walls; the walls were curved vertical structures up to 61m in length; the disc system used horizontally-set concave bowls designed for use in pairs as aircraft passed overhead to measure speed. At their most sophisticated, the devices could identify the sounds of surface vessels or aircraft up to 25 miles (c40km) away. Research into acoustic early warning was carried out in a number of countries during the early 20th century. British experiments at the Royal Flying Corps research establishment at Farnborough tested parabolic acoustic sound reflectors of varying shapes and curvature, and led to the first true sound mirror at Binbury Manor in the summer of 1915, a circular disc cut directly into a low chalk cliff. The first operational acoustic reflectors were a pair of adjustable mirrors erected on the Kent coast in 1917, followed by a series of concrete static mirrors established on the north east coast later in World War One. Further experiments were carried out after the war. This led to the building of a complex chain of mirrors on the Kent coast around Hythe in the late 1920s. Unrealised plans were also drawn up for an ambitious scheme to be installed around the Thames estuary. Acoustic devices always remained susceptible to interference from extraneous noises and adverse weather. As aircraft performance increased, the time between detection and arrival of enemy aircraft rapidly shortened and reduced the value of acoustic devices as an early warning system. By 1936 the technology of radar had replaced acoustic methods as the main form of early warning, although acoustic systems remained in use at anti-aircraft and searchlight batteries, and as backup systems in the event of radar being jammed. A national survey of acoustic early warning devices has identified only around 11 sites where remains of acoustic detection survive. Field evidence of this important aspect of the 20th defence of Britain is thus rare and all surviving examples are considered to be of national importance.

The World War I early warning acoustic mirror 335m north east of Kilnsea Grange is one of only four known surviving examples in the north east of England, and the only known example in the country that still retains its sounding column. The device survives well and makes a significant contribution to the study of early 20th century defences in England.


The monument includes an early 20th century military early warning device known as an acoustic mirror. It is located on gently undulating land 400m inland from the coast on the eastern side of the peninsular of land forming the north side of the Humber Estuary.

The acoustic 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 important industrial complexes in the north east from ships and Zeppelins during World War I. Little is currently known of the history and development of this particular system and it remains something of an enigma. Successful experiments in acoustic detection date from 1915 and the Kilnsea acoustic mirror is thought to have been built in 1918. It faces due east and was positioned to cover the northern approaches to the Humber estuary and consequently the major industrial port of Hull some 25km to the north west. Hull was a major target during both world wars and suffered significant civilian casualties during both conflicts. Between June 1915 and winter 1917 the city suffered eight air raids by Zeppelins which left 52 people dead, a total only exceeded by London.

The area of the north east coast around Kilnsea played an important part in the defence of Hull and surrounding areas during the 20th century. A variety of artillery and radar installations were located in the area, remains of which still survive today.

The Kilnsea acoustic mirror is 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 has a half hexagon shaped profile and unlike the other examples in the north east does not have supporting side walls. On the inclined face there is 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 is 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 is the `sounding column', a concrete plinth which supports 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. The mirror is Listed Grade II.

MAP EXTRACT The site of the monument is shown on the attached map extract. It includes a 2 metre boundary around the archaeological features, considered to be essential for the monument's support and preservation.


The contents of this record have been generated from a legacy data system.

Legacy System number:
Legacy System:


Books and journals
Sockett, E W, 'Yorkshire Archaeological Journal' in Yorkshires Early Warning System 1916-1936, , Vol. Vol 61, (1989), 181-188
Upton, D, 'The Yorkshire Archaeological Journal' in The Bombing of Yorkshire 1939-1945, , Vol. vOL 59, (1987), 159-174


This monument is scheduled under the Ancient Monuments and Archaeological Areas Act 1979 as amended as it appears to the Secretary of State to be of national importance. This entry is a copy, the original is held by the Department for Digital, Culture, Media and Sport.

End of official listing

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