Entering from the South Gate, The first key points of cultural interest caught my eyes is the Circular Mound Altar, which was used to worship Heaven at the winter solstice. Three levels of terraces lead to the top of the altar. You access each of them climbing a flight of 9 steps. Why 9 steps? Because according to ancient Chinese philosophy, Heaven consisted of nine layers, and the Emperor's abode was on the uppermost tier. In the center of the alter lies the "God's Heart Stone", where the emperor used to stand to say his prayer. The stone is peculiar in that it is characterized by a specific acoustic phenomenon: voice echo. It made the emperor's voice clearer and louder, thus adding to the mystic atmosphere of the service. I tried it on the stone as if I were an Emperor!

Continue northward, I reached two structures with peculiar acoustic features. One is the Echo Wall, the other the Three Echo Stones. My friend and I made the experiment: any mere whisper at any point close to the Echo Wall can be heard clearly on the other side. This is made possible because the wall is round and hermetically constructed with smooth, solid bricks. As for the Three Echo Stones, if you stand on the first stone and call out or clap your hands, the sound will echo once; on the second one, the sound will be heard twice; and on the third one, the sound will repeat three times. What an interesting play. Even though I am not a scientist, I would appreciate the wisdom of ancient Architects.