Probably the only thing The ProdigyTacita Dean, and military defence enthusiasts have in common, sound mirrors, or ‘listening ears,’ were huge concrete concavities designed to detect enemy aircraft during the First World War. Conflict’s tendency to accelerate technological advancement was fuelling German bombing campaigns, which had inched their way over the Humber and the Thames estuaries, reaching the capital at the end of May 1917. Denge, a former RAF site near Dungeness in Kent, houses the best-preserved examples of the mirrors: two hemispherical concrete bowls – 20 and 30 feet in diameter – and a 200 foot long curved wall or ‘strip mirror’ that dominates a concrete forecourt, all poised toward the open sky. Other designs for similar ‘sound collectors’ could be rotated with wheels and pedals, or mounted on cars, to give the user more control. Some of the more hilarious iterations can be seen here. I particularly recommend Professor Mayer’s ‘Topophone’ and the Japanese ‘War Tubas.’

Completed too late to be used in the First World War, and obsolete by the onset of the Second (with the invention of radar and the improvements to aircraft speed) the mirrors were abandoned almost as soon as they were finished. They’ve stood ever since as relics and ruins, jutting out of the bleak landscape like otherworldly exercises in geometry. Though their scale and material looks unyieldingly industrial, there’s something about the simplicity of the technology and the greyscale concrete that eases them into the surrounding gravel pit, and they insist on the eye a stillness that’s poised between anxiety and elegance.

The Denge mirrors worked by concentrating sound waves towards a single central point, where a microphone would be positioned. The low

engine hum of the approaching planes was amplified for an operator, who would be positioned either in a bunker below the mirror or – for the larger mirrors –­ in a control room nearby, his ears straining down a stethoscope to identify significance from the silent landscapes in which the mirrors were placed. The sensitivity required must have been an enormous strain. Test results suggested that the operators should listen for no longer than 40 minutes before they alternate, to avoid ‘irritation’, their task burdened by the ‘noise’ made by the smallest changes in the weather, or the ships in the Channel.

In many ways the mirrors provide the sensory opposite to the experience of that of battles fought between trenches. Their operators worked against a soundscape of silence rather than shelling, for short concentrated periods, rather than drawn out campaigns of attrition; and, instead of contributing to assaults on known, fixed positions, they had to be constantly on guard against an enemy that might never come. When it did, it would be distant, inhuman and mechanised – conflict conducted at one remove. Paul Virilio writes in War and Cinema that ‘weapons are tools not just of destruction, but also of perception – that is to say, stimulants that make themselves felt through chemical, neurological processes in the sense organs and the central nervous system, affecting even the perceptual identification and differentiation of objects.’ Hearing is characterised by its resistance to differentiation: eyes can be closed, the most sophisticated palates cleansed, but we never stop hearing. Even in our sleep, the brain is unable to tune out completely, and our sonic environment continues to influence dreams and the quality of rest. Sonic architecture is only now attracting attention, as the hitherto ignored influence of sound is beginning to be appreciated and incorporated into the ways architects design the spaces in which we live and work (Julian Treasure gave an interesting TED talk on this.) The experience of listening is also a spatial calculation. We infer distance by triangulating the position of a sound’s source with the position of our ears. Likewise, stereo and surround sound are able to suggest three-dimensional space by placing the hearer between two points of emission.

It’s these properties that make sound so evocative, and the absence of it so uneasy. Silence is famously deafening, whilst a sound is necessarily the sound of something, and that something may not be what you think it is, especially in the hypersensitive home fronts of global warfare. The sound mirrors at Denge were a technology that sought to intervene in the space between cause and effect, between sound and source, and confront fear of the unknown with an understanding of the nature of the connection between the two. When the mirrors were developed the threat of air assault was still a novel one, and their design betrays a very real anxiety towards this newly technologised kind of warfare. They’re angled as though they know something’s coming, but not what or when. Looking at them now, almost a century after they were constructed, they still look open, still expectant, ready for the next uncertain threat.