Dave Collyer, a member of the Kent Aviation Historical Society, said: 'We lost one of the mirrors, on the Isle of Sheppey in the 1950s due to erosion, another at Greatstone has fallen over after being undermined by gravel extraction, and a third at Lydden Spout near Dover was demolished because the local authority thought it was dangerous.'
The sound mirrors were all placed on cliffs looking out towards the continent where they provided an early warning of approaching enemy aircraft. They are about 20 to 25ft high and as much across, with a concave spherical surface. Their large size and strong shape gives them a sculptural impact apart from their interest as monuments of early military technology.
A microphone was moved about in front of the concrete mirror while a man with earphones listened to the sound. By noting the position of the microphone where the sound was loudest, the direction and height of approaching aircraft could be calculated.
'The first two sound mirrors were built near Dover in 1917,' Mr Collyer said. 'They could pick up the sound of the Gotha long-range bombers which raided London during the First World War. More of the mirrors were built between the wars. The idea was to have a chain of them round the coast. In practice their maximum range was 20 miles. They were useful for finding the direction, but they were never accurate in obtaining the height or size of the attacking force.'
Mr Collier knows of only four left in Kent, one near Spurn Head, Humberside, and two near Redcar in Cleveland. One of the mirrors, at Greatstone, near Hythe in Kent, is a 200ft-long wall mirror built in a parabolic shape, but the others are spherical in shape. Some, including one of the parabolic type, were also built in Malta where one is now part of a pig farm.
'The sound mirror technology became obsolete as bombers became faster, but the idea was kept alive as a cover for radar,' Mr Collyer said.
'The press were called in to photograph them. And when the Germans started jamming our radar, attempts were made to use the sound mirrors again. We think that preservation orders should be put on them and plaques to explain to people what they are. They are part of our history.'