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:
courtyard cannot be too noisy.
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.
Click here to see the map of the Temple of Heaven