The famous Echo Wall at the Heart of Heavenly Stone
The Heart of Heavenly Stone lies on the center of the Circular Mound Altar. In the past, when emperors delivered ceremonial speeches here a magical thing happened: even if the emperor spoke quietly, his voice sounded so powerful it could be heard clearly by the whole audience. The emperors interpreted this phenomenon as evidence that they were TianZi (the sons of God), and proclaimed this mysterious phenomenon meant everyone must obey them or run the risk of being an enemy of God.
Of course, there is a scientific reason behind this phenomenon. Sounds from the Heart of Heavenly Stone are rapidly echoed by the surrounding stone walls, and the loud voice heard by emperors and officials is actually a mixture of the original voice and echoes. But because the distance between the Heavenly Center Stone and the stone walls is short, the original sound can hardly be distinguished from the echo so people heard a louder voice.
A new layer of stone was added to protect the original which was eroded by large numbers of tourists, this changed the acoustics of the Circular Mound Altar. These days there's usually a crowd of people standing between the Heart of Heavenly Stone and the stone walls so visitors can barely hear the echo. But it's still a wonderful experience to have a chance to stand where emperors once stood and hear your voice go straight up to heaven.