The Echo Wall
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.
Iit 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.