This is old tech. Not really old tech. Just old tech. Pre radar. Pre World War II. In fact, these listening devices were developed during World War I to detect enemy aircraft approaching England.
These weird and wonderful half Death Star-like spheres used sound waves and microphones to detect German aircraft which gave the RAF planes just enough time to scramble and mount a defence.
Radar made these sound mirrors, aka listening ears, obsolete but they stand hidden in the remoteness of the Kent countryside as a reminder of the past.
The area has a desolate feel to it. In the background stands the Dungeness nuclear power station. Step out onto the sandy beach at Romney Sands and the tide can be a mile away with people making the most of the space to land surf.