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.

"One of his recurring jokes has always gone along the lines of: 'It's not rocket science, I should know, my Dad was a rocket scientist.'"

Initially Pettet-Smith was drawn to the family connection, but after researching early aircraft defence experiments, he became fascinated by the story of the sound mirrors.

I began to think more and more about the relationship between art, science and the creative process. Experimentation and ultimately failure are an intrinsic commonality of all three.

"The sound mirror experiment, this idea of having a chain of concrete structures facing the Channel using sound to detect the flight trajectory of enemy aircraft, was just that - an experiment. They tried many different sizes and designs before the project was scrapped when radar was introduced.

"The science was solid, but aircraft kept getting faster and quieter, which made them obsolete."

Pettet-Smith used an old wooden large format plate camera to record the structures, partly because he wanted to use technology that was around at the time, and secondly as it allowed him to correct the perspective of the structure in-camera without resorting to manipulation at a later date.

"Some of the structures were removed by local councils; many more were planned but never built. This series is a celebration and a cataloguing of all the remaining examples."