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.
Details
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.
Legacy
The contents of this record have been generated from a legacy data system.
Legacy System number:
35482
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
