Summary
An early-C20 military early warning device known as a sound mirror.
Reasons for Designation
This early-C20 early warning acoustic mirror on Namey Hill is scheduled for the following principal reasons:
* Rarity: one of only 11 sites identified nationally where remains of acoustic detection survive;
* Survival: a well-preserved example, whose importance is enhanced by the fact that it is thought to retain the buried remains of its operator's trench;
* Potential: the only known survivor of a former chain of similar First World War devices on the north-east coast, which will contribute to our understanding of early-C20 acoustic detection systems in England.
History
The use of aircraft as offensive weapons was a significant C20 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 (about 40km) away.
Research into acoustic early warning was carried out in a number of countries during the early C20. British experiments at the Royal Flying Corps research establishment at Farnborough, Hampshire tested parabolic sound reflectors of varying shapes and curvature, and led to the first true sound mirror at Binbury Manor, Kent 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 the First World War. 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.
This example at Namey Hill 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 the First World War. Little is currently known of the history and development of this particular system. 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 on Namey Hill is thought to have gone out of use in 1932.
Details
PRINCIPAL ELEMENTS: an early-C20 military early warning device known as a sound mirror.
DESCRIPTION: the mirror is located on a gently sloping hillside 2km inland from the coast on the block of land between the Tyne and Wear estuaries. It 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 operator's trench is currently uncertain, but is thought to be situated on the eastern side. 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. On the northern face of the mirror there is an interpretation plaque.
EXTENT OF SCHEDULING: this includes the sound mirror and a margin of 5m on the eastern side in which remains of the operator's trench may survive, and a margin of 3m on the remaining sides for its support and preservation.
