World War I early warning acoustic mirror 60m east of Boulby Barns Farm


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


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The building or site itself may lie within the boundary of more than one authority.

Redcar and Cleveland (Unitary Authority)
National Park:
National Grid Reference:
NZ 75363 19113

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: a 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; the walls were curved vertical structures up to 61m in length; the disc system used horizontal 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 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.

This sound mirror is one of four known surviving examples in the north east of England. It is the only one where the location and earthwork remains of the listening trench survive and important evidence of how the mirror operated in the field will be preserved. The mirror 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 a sound mirror. It is located approximately 500m inland from the coastal cliffs on the north east of the North York Moors.

It was built on the highest part of the north east coastline in an area used over time for early warning, ranging from a 16th century Armada beacon to a World War II radio station. The mirror was part of a chain of similar acoustic devices located on the north east coast extending from the Tyne to the Humber. 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 to 1915 and the Boulby mirror is known to have been built in 1916. It faces NNE and was positioned to cover the southern approaches to the Tees estuary. Analysis of the details of the structure of the mirror show that it was designed primarily to cover air traffic. It was used specifically to provide early warning of Zeppelin attacks on the Skinningrove Iron Works 5km to the north west. The Skinningrove works were bombed many times because at the time they manufactured high explosives and later in the war, mustard gas. There were at least two other mirrors known to cover the Tees estuary. One was located at High Springwell 17km away to the north of the estuary and was orientated to cover the north eastern approaches. This was demolished in the 1960s. The other mirror is located on the low lying land at the mouth of the Tees estuary and is protected as a separate monument.

The Boulby mirror is a `U'-shaped concrete built structure comprising a thick wall tapering in thickness with an inclined face with a shallow concave bowl shaped into its centre. On either side of the wall are projecting flanking walls, which helped to protect the reflector from noise interference and also supported the structure. The reflector is a smooth bowl measuring about 4.5m in diameter. It is not however a true circle and is slightly elongated towards ground-level. The mirror is angled to as much as 40 degrees to the vertical. The rear wall of the mirror is 5.2m in length and is approximately 4m high. The two flanking walls are 3.9m long and are slightly splayed from the mirror face and measure 5.8m from wall to wall. On the top of the rear wall there are two stone projections which have been interpreted as supports for a gantry to hold a pair of microphones which would have provided the listener with a more accurate `bi-aural' response. On the outside of both the flanking walls there is triangular shaped scarring which appears to be caused by a structure standing adjacent to the walls before the concrete rendering was added. The purpose of this is currently unknown.

In the front of the mirror there is an earthwork up to 4m wide and 0.5m high lying approximately 8.5m from the face of the structure. The ground in front is slightly scooped and this is thought to be the remains of the trench in which the listener would have sat. The ground between the mirror and the earthwork may contain significant remains of the listening facilities such as power cables, microphone and telephone lines. 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


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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