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 century defence of Britain is thus rare and all surviving examples are considered to be of national importance.
Despite some damage in the past, the three acoustic early warning devices at Greatstone-on-Sea are well preserved. They are of considerable historic interest as one of the earliest forms of airborne early warning system, following the development of aerial warfare in the early 20th century. The devices express a distinct stage of technical innovation in aircraft detection, which preceded the development of radar. They also have group value as a unique collection of the three main types of acoustic devices used in the early 20th century.
The monument includes three acoustic early warning devices surviving as upstanding and below-ground remains. It is situated on a man-made island surrounded by a lake formed by gravel extraction on the Kent coastline at Greatstone-on-Sea.
The acoustic devices form a row from north to south and are positioned to pick up aircraft noise from the east, across the Strait of Dover. The southernmost acoustic early warning device is a sound wall formed by a vertical concave reinforced concrete about 61m in diameter and 7.9m high. It is fronted by a near horizontal, gently sloping, concrete apron. The sound wall is supported by a succession of triangular concrete buttresses to the rear. Microphones were originally mounted at the foot of the horizontal apron, picking up sound reflected off the wall. A listening room was also situated to the rear of the sound wall but no longer survives. The central acoustic early warning device is a sound mirror formed of a circular concave concrete 'dish' about 6.1m in diameter. It is supported on a freestanding block of concrete. The concrete pillar which originally supported the listener's platform is no longer present. The northernmost acoustic early warning device is another sound mirror formed by a hemispherical concrete 'bowl' or 'dish' about 9.1m in diameter. It is angled skywards and supported at the back and sides with vertical reinforced concrete webs. A listening chamber is situated beneath the mirror and reached by steps. The iron antenna for the sound trumpet is still in place, at the front, in the centre of the mirror.
The three acoustic early warning devices at Greatstone-on-Sea were built following the establishment of the Acoustical Research Station at the Roughs, Hythe by the Air Ministry in 1922. They were built as part of an experimental system to detect aircraft by amplified sound. The grouping was the southern end of a sequence of mirrors along the Kent coast to Dover. Other examples survive at Hythe and at Abbots Cliff. The three acoustic devices were built sequentially over a period of about four years. In 1927-8 the smallest sound mirror was built and in 1929 the larger hemispherical mirror was constructed. Finally, the sound wall was added in 1930. The devices worked by focussing and collecting the sound waves of an aircraft engine striking the concrete mirror in a metal trumpet connected to a stethoscope worn by a listener. For the smaller mirror the listener was positioned at the front but for the hemispherical mirror a listening chamber was provided below. By moving the trumpet and recording the angle of best reception of sound the direction to the target aircraft could be established. This was then matched with the sound taken from another coastal sound mirror to establish the position and height of the target aircraft and track it in flight. The sound wall used a number of microphones at the front of the wall; the microphone with the best reception indicating the angle to the target. Following the detection of an enemy aircraft, fighter interceptors could be ordered airborne to counter the threat. The system was tested with varying degrees of success during a number of air defence exercises in the early 1930s, until it was rendered obselete by the advent of radar. In favourable conditions the acoustic devices could pick up aircraft sound between 8 and 24 miles away.
Sources: Kent HER TR 02 SE 12. NMR TR 02 SE 12. PastScape 462809.