North of the Circular Mound Altar is the Imperial Vault of Heaven, which was originally built in 1530 and rebuilt 1752. Its structure is made from wood and brick with a blue tiled roof that is topped with a gilded ball. It is 19. 5 meters high and 15.6 meters in diameter.
Built entirely of wood, the vault is supported by eight pillars. The roof has no beams but only a great number of brackets entwined within each other. The ceiling tapers upward to form a beautiful caisson.
The Imperial Vault of Heaven is better known for the wall that surrounds it. This wall is called “Echo Wall” or “The Whispering Wall”. It is 3.72 meters high, 90 centimeters thick and 65.1 meters in diameter.
The eaves of the wall and the hermetically laid bricks make wireless communication possible between two people who speak in normal voices.
To be the most effective, a couple of conditions must exist:
First, the courtyard cannot be too noisy.
Second, both people must face north
So it is better to stand by the wall at the back of the two side chambers. The entrance of the courtyard prevents the voices from being heard by others.