In front of the Imperial Vault of Heaven is the Triple-sound Stone,also called Sanyinshi in Chinese. If one person stands on the first stone and speaks toward the hall, his sound can be heard once; if he does the same thing on the second stone, then his sound will echo twice; and if he speaks on the third stone three echoes will be heard.

Reason:
The reason is similar to that for the above-mentioned Echo Wall; but this time the echoing sound waves are triggered simply by different distances from the stone to the hall.

Beijing Tour Routes covers Temple of Heaven:

Temple of Heaven Photo