Echo Wall

Echo Wall
 
Echo WallThe Echo Wall encloses the Imperial Vault of Heaven. Its perimeter is 193 meters (633 feet). The wall is flat and smooth, which allows sound waves passing smoothly. If you and your friend stand by the wall behind the East Annex Hall and the West Annex Hall respectively and you whisper a word, then your friend will hear clearly what you said. The phenomenon utilizes the theory of sound waves, resulting in a "mysterious atmosphere".