Listening for the Enemy: Giant Ears on the British Coast

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The enormous concrete shells are as tall as a house, and they were designed to listen for enemy aircraft in the 1920s and 30s. Pre-World War II acoustic experiments led to some extraordinary architecture -- and to a remarkably effective technology which ultimately succumbed to the invention of radar.

Photo Gallery: Britain's Forgotten Sound Mirrors Photos
Robert Riddle

In the early summer of 1934, the rolling green hills of the county of Kent were no longer as contemplatively idyllic as they once were. For nearly 12 years, William Sansome Tucker had been coming to the lonely coastal area in the southeast of England to attend to his experiments -- but in those days of May, his role had changed into that of an event manager rather than a scientist. New visits were constantly being arranged, and the physicist was fully occupied organizing the arrival and departure of his guests.

They included senior officials from the Air Ministry as well as well-known scientific colleagues such as the Scotsman Robert Watson-Watt, all of whom wanted to see for themselves the progress being made in the field of acoustic detection. Above all, they wanted to see the imposing technology that had made it possible to track planes in the sky before they were detectible to the human ear or, astonishingly, even the eye.

The traumatic experiences suffered during World War I were all too fresh in British minds; memories of German bombers and airships dropping their destructive payloads over the country's cities. The lack of an effective air defense had been a major weakness for the UK during the war. Now the Nazis were in power in a newly-rejuvenated Germany, and nothing good could be expected of their expansionist plans. Should there be an attack on Britain, it would come, the military believed, from the skies. The detection and tracking of enemy aircraft was therefore a top priority.

A visit to the research facility near the Kent town of Hythe gave the senior officials a feeling that a solution could be close at hand. Particularly when they saw the mysterious early warning system, which had been manifested near pebble beaches in the form of monumental reinforced concrete shells. They were positioned along the coastline facing out into the English Channel like oversized ears.

"Sound Mirrors"

Tucker called them "Sound Mirrors", and they were able to amplify the noise of engines on incoming aircraft by reflecting sound waves off their curved surfaces and concentrating them to a focal point, like rays of light in an optical lens. Based on the spot where the sound was loudest, calculations could be made to determine the direction of the plane.

Noise amplifiers using this principle had already been used in France and England during World War I in efforts to develop a means of locating enemy artillery. Among other things, parabolic mirrors were used which could be guided by a movable joint. From experiments it was known that for low frequencies, like the hum of aircraft engines, large surface-areas tended to be best.

In 1915, a British researcher had blasted a plate-like expanse five meters in diameter in the limestone cliffs on a headland south of the River Thames for just such a trial. Using a stethoscope as a kind of ear trumpet, he listened to the sounds being reflected. The horn was on a swivel mount and was connected to a pointer, which made it possible to calculate the bearing.

Although at that time no planes came close enough to be located, the functionality of the limestone mirror was nonetheless demonstrated later on: during the war, the British government commissioned a number of scientists into military service -- including physicist William Sansome Tucker, fresh from attaining his doctorate in 1915, who during a German attack on London in the spring of 1918 was part of the team operating the acoustic early warning station on the limestone cliffs of Kent.

Sufficient Test Subjects

Tucker remained in the Royal Engineers after the war and, in 1922, he took charge of acoustic research at the newly-established Air Defense Experimental Establishment. A search for a suitable location for future experiments led to the area around Hythe on the south coast of England -- not least because it was under the flight path of commercial aviation heading towards France, thereby providing sufficient test subjects.

It was there that, by the end of the 1920s, the scientists had erected a total of five concave mirrors made of steel and concrete, each 20 to 30 feet (six to nine meters) high and with different bowl depths. Tucker, however, wanted more. The 30-foot mirrors were very efficient for waves up to three feet, he found, but the sounds that interested them had lengths of 15 to 18 feet. That, in turn, meant that the mirror surface needed to be made 10 times larger.

When the sixth mirror was completed in 1930, it had surpassed all previous records in its dimensions: A curved wall 60 meters long and eight meters high. And it wasn't only the dimensions which were new, but also the type of listening device. In addition to conventional listening posts in front of the wall, Tucker used one of his earlier inventions, a power-operated hot-wire microphone. It worked as an indicator of sound waves: When the waves hit the heated wire, it cooled off, changing the electrical resistance and thus the strength of the electrical current which heated the wire up.

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BTraven 06/03/2011
Astonishingly, the concrete ears seem to fit in the landscape perfectly. It makes it more attractive. A touch of mystery has never done any harm to areas no matter unspoilt or urbanised.
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