World War I early warning acoustic mirror on Namey Hill, 570m north of Carley Hill Cricket Ground


Heritage Category: Scheduled Monument

List Entry Number: 1020325

Date first listed: 07-Nov-2001


Ordnance survey map of World War I early warning acoustic mirror on Namey Hill, 570m north of Carley Hill Cricket Ground
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The building or site itself may lie within the boundary of more than one authority.

District: Sunderland (Metropolitan Authority)

National Grid Reference: NZ 38950 59609


Legacy Record - This information may be included in the List Entry Details.

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.

The World War I early warning acoustic mirror on Namey Hill, 570m north of Carley Hill Cricket Ground, is one four known surviving examples in the north east of England. It survives well and makes a significant contribution to the study of early 20th century defences in England.


Legacy Record - This information may be included in the List Entry Details.


The monument includes an early 20th century military early warning device known as a sound mirror. It is located on a gently sloping hillside 2km inland from the coast on the block of land between the Tyne and Wear estuaries. 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 it is thought that the Tees/Tyne early warning system dates to the last two years of the war. This mirror faces east and was positioned to cover the approaches to the Tyne and Wear estuaries. There were probably other mirrors as part of the Tyne and Wear defensive chain but the location of these is currently unknown. The mirror is a `U'-shaped, concrete built structure comprising a thick wall with an inclined face and 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 4.5m in diameter, inclined approximately 11 degrees to the vertical. The rear wall is 5.8m in length and is 4m high. The two flanking walls are 3.9m long. The reflected sound was detected by a microphone placed in front of the dish and then transmitted to the headphones of the operator who sat in a trench to the front. The location of the operators trench is currently unknown. It has been suggested that at this mirror the microphone was secured in front of the dish by wires attached to the side walls, so allowing it to be variably positioned. This differs from other mirrors in the north east where the microphone was fixed on a metal post in front of the dish. The monument also includes a margin of 5m beyond the mirror on the eastern side in which remains of the operators trench may survive and a margin of 3m on the remaining sides for the support and preservation of the monument. On the northern face of the mirror there is an interpretation plaque which is included in the monument. The mirror is Listed Grade II.

MAP EXTRACT The site of the monument is shown on the attached map extract.


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

Legacy System number: 34835

Legacy System: RSM


Books and journals
Sockett, E W, 'Durham Archaeological Journal' in A Concrete lLstening Mirror At Fulwell, Sunderland, , Vol. VOL 6, (1990), 75-76

End of official listing