Fulwell Acoustic Mirror

Remembrance Day feels like an appropriate time to look back at an unlikely piece of military history on Wearside. The Fulwell Acoustic Mirror might not look like much, but the stark concrete slab played a key role in air defence 15 years before Radar was implemented.

Today, bombing raids over Britain are mostly associated with World War II. The blitz, and associated ‘blitz spirit’, have become an integral part of our national identity. Not many remember how World War I brought warfare from the skies for the first time in history. Zeppelins, the new technology of the day, were used to bombard England’s North Sea coast – and the region’s shipbuilding and coal-mining centres were prime targets.

In 1913, Sunderland was reckoned the biggest shipbuilding town in the world. The County Durham coal field achieved a record yield in the same year. For the German military, the chance to strike a blow at Britain’s industrial prowess and potentially curb the effectiveness of the fearsome imperial navy was too tempting to ignore. Although aerial warfare was a new and untested tactic, Zeppelin raids hit the northeast of England.

By 1916, this was becoming a serious problem. An April raid over the Wheatsheaf area of Sunderland, just to the north of the Wearmouth Bridge and close to the shipyards and Wearmouth Colliery, killed 22 people and injured 100 more. Although Zeppelins were big, slow-moving and vulnerable, there was little notice of their arrival. And once the bombs started falling, the damage was done.

Ingenuity was called for. British forces in Belgium set up an Experimental Sound Ranging System at Kemmel Hill. The boffins, led by Lawrence Bragg and William Sansome Tucker, perfected a system for calculating the position of enemy guns using microphones. This acoustic technology owed much to the sound effects created by domed buildings - think of the whispering gallery in St. Paul’s Cathedral. Over time, this could be adapted to “spot” the drone of a Zeppelin with large slabs of concave concrete and a carefully positioned microphone mounted on a plinth in front. The underlying idea of reflecting sound waves is not too dissimilar from a satellite dish or radio telescope.

Acoustic mirrors sprang up along England’s east coast in 1917. A soldier would be stationed in a bunker in front of the mirror, listening for any sound of incoming threat. Once operational, the system offered 15 minutes warning of an air raid: time enough to alert the local authorities and attempt to evacuate the area.

The Sunderland acoustic mirror, at Fulwell quarry, is one of several that are known to have been built along the coastline. In the northeast, it has neighbours at Seaham, Hartlepool and Redcar. Together, they form the top end of a chain that stretches down as far as Dover and Dungeness. Much like the beacons of old, set ablaze atop each hill to warn of armies on the march, these briefly fulfilled a valuable defensive function.

There were likely many more that have been lost. By the 1930s, the increased speed of aircraft and advances in radar rendered acoustic mirrors obsolete. As a child, I remember visiting Fulwell Quarry but heard nothing of its concrete mirror, buried in the undergrowth. Photos from 2003 show it half hidden by bushes, semi concealed the long grass. Few recognised what it had been, so few cared to preserve it. More recently, after taking on the local parkrun, I caught up with the story.

The centenary of the outbreak of WWI in 2014 brought renewed interest in the Great War and its heritage. That interest helped encourage the restoration of the surviving sound mirrors, including Sunderland’s. Today, it’s easily accessible from the old quarry and the allotments. The site has been tidied up, and there’s signage to explain what it was all about. The restoration wasn’t perfect; the original blue plaque described it as a convex dish when, in reality, it is concave. But, more than 100 years later, the mirror still stands as an unexpected monument to an unlikely side story of the First World War.

Further reading: Northeast Heritage Library, which has some historic photos, Andrew Grantham’s website with a more detailed history of the Fulwell mirror, and his related Twitter account for all things whispered into concrete ears.