Before the application of radar became a reality a number of experimental early warning systems were developed by the British military. One which showed the most promise – as it actually worked – were the acoustic mirrors built at Denge on the south coast of England. Quickly superseded by radar they were abandoned but still remain at their post, obsolete concrete leviathans on an island in the middle of a nature reserve, reminders of a dangerous time in European history.
The mirror would pick up the sound of any aircraft approaching the coast of the UK. If they were not scheduled (and flights in and out of the country were closely monitored) then they could be judged as possibly being enemy aircraft. Sound waves were caught in the focal point of the mirror and relayed though microphones to an operator, who could then alert the appropriate authorities. The mirrors were able to give a fifteen minute warning of an approaching assault on the England.
The acoustic mirrors were developed by Dr William Sansome Tucker who built a number of installations along the coast, of which the Denge mirrors are by far the best preserved. Altogether there were three mirrors built at the Denge complex, out of which the 200 foot curved wall achieved the best results.
Ironically, the structures that fascinate contemporary visitors are the circular dishes, one of 30 feet across and the second of twenty. Perhaps this is because their shapes resonate with our own time and experiences. However, although their range was comparable, they were not as accurate as the 200 foot Goliath which accompanies them.
Length may not always be considered important, but it was calculated that to provide greater accuracy, especially in the vertical plane, then the length of the mirrors would have to be increased and increased up to ten times. Its sheer size prohibited the use of a portable sound collector so static listeners and microphones were used instead.
The long mirror was complete by 1930. In a typically thrifty British manner, the microphones were stored in a shed to begin with but then a two story listening chamber was constructed behind the mirror. Although radar was invented in 1932 the mirrors participated in the yearly Air Defense of Great Britain exercise in conjunction with the Royal Air Force until the middle of the decade. The long mirror was able at peak efficiency to detect incoming aircraft at a range of over twenty miles, with thirty miles being the longest distance measured.
This was a problem. Although a significant breakthrough in terms of detecting aircraft, engineering developments meant that planes were getting faster all the time. The use of the mirrors was questioned, as by the time the aircraft were located and messages about their whereabouts relayed and action taken against them, they could already have reached their targets.
Yet although the mirrors were never used in the Second World War their early use was pivotal to the creation and then improvement of a centralized reporting and command structure for the RAF which was used in the oncoming war. The need for an automatic transmission of results was also acknowledged and again this system was used during the Second World War.
Plans were afoot for a sequence of mirrors along the south coast which would preliminarily protect the Thames Estuary, the river system leading to the English capital, London. In 1935, government approval for the scheme was sought, given and sites were selected. Yet all over the world, radar was being researched and discoveries applied, nowhere less than in the UK.
In February of 1935 Robert Watson-Watt had begun his preliminary experiments with radar at Daventry. The very next month a radar research station was up and running at Orford Ness. Four months later the new radar equipment was detecting aircraft at a range of 40 miles. The age of the acoustic mirror was over before it had properly begun.
War came to Great Britain in 1939. By then the Acoustical Research Station at Denge was closed down. The mirrors were then used to experiment on the effect of explosive charges at their foci. Later the War Department ordered that they should all be blown up. Fortunately for us this did not happen – any use to an enemy occupier was deemed negligible and so their obliteration was judged too expensive and unnecessary.
The mirrors at Denge stand to this day. The listening chamber was deemed unsafe and demolished but the 20, 30 and 200 foot mirrors persist. They stand as reminders of a time when Britain was preparing for imminent attack from a seemingly invincible enemy. They played a small part in the gargantuan effort against an insidious ideology which would, had it succeeded, have changed Europe undesirably forever.