The concrete blocks that once protected Britain

More than 100 years ago acoustic mirrors along the coast of England were used to detect the sound of approaching German zeppelins. The concave concrete structures were designed to pick up sound waves from enemy aircraft, making it possible to predict their flight trajectory, giving enough time for ground forces to be alerted to defend the towns and cities of Britain. Invented by Dr. William Sansome Tucke and known as sound mirrors, their development continued until the mid-1930s, when radar made them obsolete. Joe Pettet-Smith set out to photograph all the remaining structures following a conversation with his father, who told him about these large concrete structures dotted along the coastline between Brighton and Dover. "When I was a child my father told me stories about my grandfather and his involvement in radar," says Pettet-Smith.