Last week, I had the rare opportunity to photograph the Denge sound mirrors (acoustic mirrors, or listening ears), now within the RSPB nature reserve near Dungeness in Kent.
They were constructed, starting in the late 1920s, at a time when there was a problem with obtaining early warning of air raids that might potentially come from across the English Channel. When they were commissioned, radar was not available, so sound mirrors were developed to detect the noises from aero-engines. A formation of bombers would emit a great deal of noise that could, in theory, be detected at long distances.
The British government installed a series of listening posts around the south and east coasts facing the continent of Europe based around large concrete parabolic mirrors. These would collect and reflect sounds, concentrating them on microphones; a trained operator could distinguish the sounds of approaching aircraft from, say, shipping. However, by the time the construction was finished in the early 1930s, radar was already making these obsolete. The listening years could only give about 15 minutes of warning, scarcely enough time to get fighter aircraft off the ground before the bombers would have dropped their loads on coastal targets. About the same time that the sound mirrors were being built, the first patents for radar were being filed: by the mid-1930s radar became a practical possibility, rendering the sound mirrors obsolete as soon as they were built.
On the site, there are three mirrors: a large 30-foot wide parabolic circular mirror; a smaller 20-foot parabolic mirror; and a 200-feet wide wall. The RSPB representative told us that the wall was the most sensitive of the three and acted as a wide-angle sound collector; the other two were for more precise detection and analysis as the target aircraft came closer.
Today they sit in states of various decay on the RSPB reserve on an island within some flooded gravel pits, surrounded by reed-beds and scrubby trees and bushes.
The 30ft and 20ft acoustic mirrors seen across the reeds
The RSPB only admits visitors to the site on some open days during winter (outside the bird breeding season). I was lucky enough to get on a special visit organised by Robert Canis with the RSPB: we had a couple of hours to photograph the relics. It was one of those cold, grey days in December when the wind blew rain horizontally. Photography could only be done with backs to the wind, except in those few moments when the rain stopped.
Anyhow, I had a productive time despite the weather, making photographs of each of the three remaining structures. FWIW, I used my Sony A7CII as my main camera with a 16-35mm lens. The 16-35 worked well because the island the mirrors are on is quite small, and, given their size, an ultra-wide lens helps. I've processed the colour pictures in Lightroom to reflect the way the day and the place felt. I had hoped to get some long exposures with an ND filter to get the movement of clouds. (The A7CII has a bulb timer which makes long exposures very easy.) In the end, though, I only got one long exposure (at the end of this post) because of rain landing on the ND filter. I've processed that for atmosphere rather than documentation.
20 foot sound mirror
The wall: a 200 foot sound mirror
The wall: a 200 foot sound mirror, rear aspect
The 30ft acoustic mirror seen through scrub and reeds
30 foot sound mirror: rear
30 foot sound mirror: the microphone stand remains
And finally…
Alien beams. 4 min exposure with Lee 15 stop IRND filter.