The Sound Mirrors of Denge
Sound Mirrors at Denge, UK. Photo by Hywel Williams, used under a Creative Commons Attribution-Share Alike 2.0 Generic license.
If a technology solves a problem that’s particularly desperate (or it solves one that’s less desperate but more profitable), it’s often prefigured by other, lesser technologies. For example, railways were huge money-makers—it’s difficult to overestimate the amount of inland trade opened up when one doesn’t have to rely on a navigable river—so the problem was attacked with canals for a solid sixty years before steam-driven cargo trains took their place. Birmingham, Manchester, and New York City all owe their initial growth to canals, not railways.
Or take the telegraph. The annihilation of distance’s effect on communications presents so many possibilities that there were several notable attempts to do it before electricity actually accomplished it. The most successful was the Chappe Semaphore system, developed by Frenchman Claude Chappe.
He built a network of semaphore towers, superficially resembling windmills, across Napoleonic France; where a windmill has vanes, these had twenty-foot semaphore arms that could be arranged in 196 different positions. Ninety-six of the position “symbols” were reserved for codes; each possible pair of codes was assigned a word or phrase—and as 96×96 is 9,216, the system was capable of supporting quite a few concepts without resorting to spelling out the message. The system was successful enough that France didn’t switch over to electric telegraphy until the late 1840s, nearly ten years after Samuel Morse and his ilk made it a practical reality.
Another pre-figured technology was radar. It’s widely believed that the UK won the Battle of Britain because their advanced radar technology let them direct their defending aircraft very effectively. That’s not so: the cavity magnetron was a great step forward, and did give the British a radar advantage, but it didn’t go into mass production until November of 1941. The Battle of Britain was over by the end of October. There had to have been something else that made the difference, and that is the kernel of a tale that leads to the sound mirrors of Denge, on the British coast of the English Channel listening almost due east across the Dover Strait.
Acoustic detection was first used seriously in World War I. The first warplanes were such tinkertoys in comparison to modern jets that it’s easy to overlook the new element of speed they introduced to the battlefield. A marching soldier moves at a few kilometers per hour. A quality horse galloping flat out can do fifty, for a short while. Early WWI planes could fly at three times that, and keep it up for hours. Prior to the development of battlefield radio only a pre-laid telegraph wire could outpace an airplane, so very often planes were overhead observing or otherwise making a nuisance of themselves before anyone underneath could possibly have learned they were coming.
By the mid-1920s area bombers were practical and people were sure they would do immense damage to cities if unopposed. Practical radar for detecting incoming aircraft didn’t exist until the mid-1930s, so in a narrow window between 1914 and about 1935, the best answer was to listen for their engines.
The main goals of any detection system were to extend the distance at which aircraft would be noticed and to learn the direction from which they were approaching. The earliest systems looked ludicrous, being essentially scaled up versions of old-fashioned ear trumpets for the deaf. The horns extended the range of the human operator’s hearing, and with two of them one could piggy-back on our natural ability to detect the direction of a sound. But from there they rapidly grew in size and became a bit like artillery—something that needed to be put on a platform and be run by a crew. Perhaps the most famous example of that stage is the wonderfully named Japanese War Tubas; that’s not their real name, but one look at them explains the nickname.
Greater performance required ever-greater size. It was the British who took the next logical step and started making stationary sound mirrors. You’re probably already familiar with the effect. Stand in an open plaza and shout in the direction of a building at its far end and you’ll hear an echo bouncing off the hard surface of the distant wall (you may need to run for it if security comes out to ask why you’re shouting at their building). If you curve the wall, you can even channel the sound and increase the range at which the source can be from the reflector. Size issues meant a person couldn’t have one of the stationary mirrors stuck on either ear and so direction-finding was lost, but a slick bit of engineering solved that problem. A parabola focuses sound best, but a hemispherical curve loses only a fraction of the range and also makes it possible to calculate the direction of the source by measuring the intensity of the echo at two locations within the arc of the curve.
In the end there were three sound mirrors built at Denge under the control of Air Defense Great Britain (one more would be built at Maghtab in Malta to protect the Mediterranean Fleet at their moorings in Grand Harbour at Valletta, though the British would get cold feet at the last minute and move their ships to Alexandria in 1939). The smaller mirrors were hemispheres of concrete, reminiscent of modern satellite dishes, one twenty feet in diameter and one thirty feet. The third was a massive wall, like a curved dam some 200 feet long. A full-scale test was conducted on the 17th and 18th of July, 1934 and the mirrors detected all the simulated attacks; the large mirror could pin down the bearing of approaching bombers to 2.5° and on a good day had a detection range of 65 kilometers. On a bad day full of noisy wind gusts, the range dropped to under ten kilometers.
With that kind of distance, the British realized that the sound mirrors would need another component: analysis and communication processes to use the results the technology was giving them. On May 1st, 1936, Air Defense Great Britain hived off its bomber aircraft and reorganized to focus purely on defensive fighting under the name RAF Fighter Command. They developed the procedures needed to react in the minutes the sound mirrors bought them, only to be gifted shortly thereafter with the Chain Home radar system that extended their senses out to 200 kilometers or more.
The German Freya radar system was more technologically advanced, but the simpler British system was completed before the war broke out and was supplemented by the extremely quick reaction times Fighter Command had developed to work with the now-obsolete acoustic mirrors. Radar stations did the grunt work by detecting attacks, their direction, and the number of aircraft involved. But then this information was passed from all stations by telephone directly to the top of their organization: one of Fighter Command’s seven filter rooms that plotted the results and passed them off to their subordinate group headquarters with instructions on what to do with their fighters. By the time the attacking aircraft crossed the coast (and so could not be seen by radar any more), the defense against them was already in position and ready to fight. Of all the things that contributed significantly to the UK fending off the Luftwaffe, the humble management system developed originally to deal with the limitations of acoustical detection is probably the least heralded.
