exture and shape - Denge Sound Mirrors, also known as Acoustic Mirrors, Concrete Dishes, or Listening Ears, are large concrete structures designed as an early warning system for Britain to detect enemy aircraft.
These worked by focusing the sound from the plane’s engine so it could be heard before it was visible.
There were three designs of mirrors, 20ft, 30ft, and 200ft, and all three can be seen in Greatstone