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.

It was built on the highest part of the north east coastline in an area
used over time for early warning, ranging from a 16th century Armada
beacon to a World War II radio station. 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 the Boulby mirror is known to have been built in 1916. It
faces NNE and was positioned to cover the southern approaches to the Tees
estuary. Analysis of the details of the structure of the mirror show that
it was designed primarily to cover air traffic. It was used specifically
to provide early warning of Zeppelin attacks on the Skinningrove Iron
Works 5km to the north west. The Skinningrove works were bombed many times
because at the time they manufactured high explosives and later in the
war, mustard gas. There were at least two other mirrors known to cover the
Tees estuary. One was located at High Springwell 17km away to the north of
the estuary and was orientated to cover the north eastern approaches. This
was demolished in the 1960s. The other mirror is located on the low lying
land at the mouth of the Tees estuary and is protected as a separate
monument.

The Boulby mirror is a `U'-shaped concrete built structure comprising a
thick wall tapering in thickness with an inclined face with 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 measuring about 4.5m in diameter. It is not however a true
circle and is slightly elongated towards ground-level. The mirror is
angled to as much as 40 degrees to the vertical. The rear wall of the
mirror is 5.2m in length and is approximately 4m high. The two flanking
walls are 3.9m long and are slightly splayed from the mirror face and
measure 5.8m from wall to wall. On the top of the rear wall there are two
stone projections which have been interpreted as supports for a gantry to
hold a pair of microphones which would have provided the listener with a
more accurate `bi-aural' response. On the outside of both the flanking
walls there is triangular shaped scarring which appears to be caused by a
structure standing adjacent to the walls before the concrete rendering was
added. The purpose of this is currently unknown.

In the front of the mirror there is an earthwork up to 4m wide and 0.5m
high lying approximately 8.5m from the face of the structure. The ground
in front is slightly scooped and this is thought to be the remains of the
trench in which the listener would have sat. The ground between the mirror
and the earthwork may contain significant remains of the listening
facilities such as power cables, microphone and telephone lines. 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: 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.

This sound mirror is one of four known surviving examples in the north
east of England. It is the only one where the location and earthwork
remains of the listening trench survive and important evidence of how the
mirror operated in the field will be preserved. The mirror 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

Source: Historic England