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.

The acoustic mirror was part of a chain of similar acoustic devices
located on the north east coast extending from the Humber to the Tyne.
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 from 1915 and
the Kilnsea acoustic mirror is thought to have been built in 1918. It
faces due east and was positioned to cover the northern approaches to the
Humber estuary and consequently the major industrial port of Hull some
25km to the north west. Hull was a major target during both world wars and
suffered significant civilian casualties during both conflicts. Between
June 1915 and winter 1917 the city suffered eight air raids by Zeppelins
which left 52 people dead, a total only exceeded by London.

The area of the north east coast around Kilnsea played an important part
in the defence of Hull and surrounding areas during the 20th century. A
variety of artillery and radar installations were located in the area,
remains of which still survive today.

The Kilnsea acoustic mirror is a concrete built structure comprising a
thick slab wall, 5.2m long, 2.5m deep and approximately 4m high tapering
in thickness to the top with an inclined face on the seaward side. It has
a half hexagon shaped profile and unlike the other examples in the north
east does not have supporting side walls. On the inclined face there is a
shallow concave bowl, approximately 4.5m in diameter and just over a metre
deep which acted as the sound reflector. The surface of the bowl is
smooth, whereas its rim and the rest of the wall surface has a rough
surface `pecked' with a pick. Approximately 1.5m to the east of the mirror
wall is the `sounding column', a concrete plinth which supports a
pyramidal column cast around a metal pipe which extends 1.5m high. This
originally supported a swivelling, trumpet-shaped `collector head', which
via two wires and a stethoscope head set, fed detected sounds to a
listener who was located nearby, usually in a below ground bunker. A 1980s
survey noted a possible entrance to an infilled trench or bunker south of
the concrete plinth which would have housed the Listener's Post. 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.

Source: Historic England

Reasons for Scheduling

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.

The World War I early warning acoustic mirror 335m north east of Kilnsea
Grange is one of only four known surviving examples in the north east of
England, and the only known example in the country that still retains its
sounding column. The device survives well and makes a significant
contribution to the study of early 20th century defences in England.

Source: Historic England

Sources

Books and journals
Sockett, E W, 'Yorkshire Archaeological Journal' in Yorkshires Early Warning System 1916-1936, , Vol. Vol 61, (1989), 181-188
Upton, D, 'The Yorkshire Archaeological Journal' in The Bombing of Yorkshire 1939-1945, , Vol. vOL 59, (1987), 159-174

Source: Historic England