The wall around the Imperial Vault of Heaven is the well-known Echo Wall (Huiyinbi). It is3.27 meters high 90 centimeters thick, and 193.2 meters long and has three doors to the south.

Interesting: What is interesting for the wall lies in the fact that if one person speaks or even whispers at one side of it, another would be able to hear rather clearly on the opposite side.

Reason: This is because of the conductibility of sound waves; the wall itself has eaves and is hermetically laid. These factors are supposed to let human voice "travel" round the wall easily.

Temple of Heaven Photo