David Bond, FT Weekend, July 2018

Forgotten, dilapidated and, in one case, buried by the local council as an eyesore; these smooth, spherical concrete structures known as acoustic mirrors provided the UK with its first early warning system against German air attack during the first world war. Invented a quarter of a century before the arrival of radar quickly rendered them obsolete, the giant concave bowls were designed to capture, amplify and focus sound to an operator. Using a listening trumpet or, some years later, a microphone, the officer could then plot the trajectory and distance of the enemy. This freed up valuable minutes to prepare air defences. With a range of about 20 miles, the acoustic mirrors were built in two clusters; one along the south and south-eastern coast of England and another along the north-east coast of the country. Some were carved into cliff faces, providing a historic echo of the now commonplace satellite dish. Others were standalone structures on cliff tops and hills.

These photographs by Joe Pettet-Smith capture the less well-known variations that developed over the years as engineers tinkered with the technology. The initial 15ft concave bowl eventually developed into wider, 200ft curved walls, which captured more sound. Two mirrors, at Dover in Kent, were covered over with earth by Kent County Council in the 1970s as part of an attempt to conceal or demolish military infrastructure left over from the two world wars. They were only rediscovered when the National Trust excavated wartime tunnels in the cliffs. The two sound mirrors there, which date from 1917, were dug out in 2014 and opened to the public a year later. Three different structures stand on an island at Denge, a former Royal Air Force site near Dungeness, accessible for just a few days every year. “The sound mirrors at Denge in Kent have become quite famous,” writes Andrew Grantham, a blogger who has a website dedicated to the sound mirrors. “But it is less well known that a number of other mirrors existed, built to a range of different designs.” Academics and military enthusiasts are split over their effectiveness. Some view them as no more than an experiment that was quickly abandoned once advancing aircraft speed significantly reduced the time gap between hearing a plane and seeing one. In any case, the invention of radar in the mid 1930s meant the UK defence ministry quickly switched efforts to building its air defences around a new technology that could detect planes at a distance of 40 miles.

Nevertheless, the structures provide physical evidence of what some experts say was the world’s first properly organised early warning air defence system. “This is the first time there was an arranged structure that could feed information back into a central system, which could then take decisions about how to respond,” says Jon Barker of the National Trust. “Sound mirrors evolved quickly. But then so did plane engine speed.”

©Financial Times 2018 - This text is for research purposes, no copyright infringement is intended.

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