Friday, 21 September 2012

Sound and vision: Hythe's acoustic mirror

On the roughs above Hythe in Kent, on Ministry of Defence land, stands a 30 foot high concrete ear. Borne on a frame of umbrella-shaped iron rods, the disc is angled towards the sky, ready to catch any sounds that come its way. The sound mirror looks out over the flat expanse of Romney Marsh, and miles out to sea (France is just 23 miles away), once assigned the task of monitoring the sky over the English Channel. In 1923, when the mirror was built, it was hoped that in the event of an attack it would pick up the engine noises of enemy aircraft out at sea; an improbable yet innovative early warning defence system. The sounds of the plane would bounce back to the focal point of the mirror, where a waiting operator would be alerted the the presence of planes. Picking up sounds up to 15 minutes before the unaided ear, this bought crucial time for anti-aircraft defences to be activated. This stretch of coast had long been on the frontline of defence against invaders, and the mirror overlooks the remains of the solid, brick-built Napoleonic Martello towers which stud the coastline below; the nearby Royal Military Canal, similarly built to withstand the threat of French attack, is just out of eyeshot.

The mirror worked on a similar concept to the modern TV receiver, except with sound waves instead of radio waves, and was the latest in a series of attempts by the military to harness the potential of sound. Experiments had started during WWI, when the possible dangers of devastating airborne attack was realised, and similar technology included sound ranging to detect enemy guns as well as listening wells. A 1916 account of tests of a sound mirror considered the invention to be a success: “A man 100m distant, reading a newspaper in a low voice was heard perfectly. Airplanes were heard up to distances of 8 kilometers.”1

Precursors to the concrete mirrors were cut directly into the chalk of the Kent hills, and there were experiments with acoustic mirrors at Hythe before the 1923 mirror; an earlier 20 foot cast concrete mirror had been built alongside a building lab, workshop, store and provisions for technical assistance to live on site. An acoustic research station was also built at nearby West Hythe.

When the potential of the sound mirrors was proven – it was claimed that they could capture up to ten times for sound than unaided ears – plans were made for lines of discs to be erected around the coast. A 30 foot high mirror was built at Abbots Cliff near Dover in 1929 and a 200 foot mirror at Denge, near Dungeness, with microphones positioned on the forecourt to capture noise, was completed in 1930 (two, smaller mirrors had also been built at the site beforehand). Building materials were carried along the coast to the latter by the miniature Romney, Hythe and Dymchurch railway, a passenger train which was itself later put to military use during the second world war.

For one reason or another – the inconveniences of wind and rain, increased noise and the advent of faster planes – the sound mirrors never saw action (like the Martello Towers and Royal Military Canal before them). They were abandoned in the 1930s in favour of radar, and orders were made for them to be destroyed.

These orders were never carried out. Today, the Hythe sound mirror has faded into the Kent hills, camouflaged into the landscape and rendered nearly inaccessible by a thicket of head-high thorns and nettles, overrun with rabbits. The mirror is slowly crumbling into the hillside and now resembles a part-eaten biscuit, with a chunk taken out of the side. The structure might have been abandoned and the technology made obsolescent, but the most striking thing about the site today is its extraordinarily rich sonic landscape. The entire hillside hums as breeze sweeps through the trees and long grass, the thistles creak in the wind and grasshoppers rub their legs together. Birds take turns to fill the air with their coded langauge: from high peep-peeps and chchchs to the woo-woo-wooing of the wood pigeon. Sheep intermittently baa in call and response. This natural background noise is occasionally punctuated by the rumbling of a distant, out of sight plane, the distant bark of a dog or the brief revving of a boy racer and sirens on the coast road below.

1. Quoted in Echoes from the Sky: A Story of Acoustic Defence, Richard N Scarth (Hythe Civic Society, Hythe, 1999), accessed in the Local Studies Unit at Hythe Libary

This recording has been submitted to the Field Recording Archive, a new initiative based in Manchester.

For more information on sound mirrors, including sound mirrors at other locations around the country (particularly on the North East coast) visit the website of Andrew Grantham.

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