English: The largest of three concrete "sound mirrors" on the coast in Denge, Kent, England, built in the 1930s for air defence. Before radar was developed in World War 2 these were used to detect incoming enemy aircraft by listening for the sound of their engines. This "sound wall" was the largest, a curved wall about 5 m high by 70 m long. The wall focused all the sound coming from a particular direction to a point in front of the wall, the focus; an observer or microphone would be stationed there. The curve of the wall had a circular shape, rather than the parabolic shape used in many sound detectors. The circular shape allowed the sensitivity direction to be swept across the horizon by moving the microphone laterally. The structure is preserved for historical interest.