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.
Details
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.
Legacy
The contents of this record have been generated from a legacy data system.
Legacy System number:
35470
Legacy System:
RSM
Sources
Books and journals
Legal
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 Culture, Media and Sport.
End of official list entry
