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The Echo Wall, a circular wall of polished bricks with an opening to the South. ^[1]Dengjiang bricks（澄浆砖）are the material being used for the Echo Wall, these high-quality bricks were produced in Linqing, Shandong Province, and are perfect for reflection of sounds. ^[2]Producing these bricks is a complex process. First, the clay needs to be sifted and mix it with water to form mud. They then let it sit for a while. Once the impurities had settled, take the upper layer of the fine mud, shape them into rectangular shapes, and fire them.^[3] The circular brick wall forms famous acoustic phenomena, the Echo Wall, the One Sound Stone, the Double Sound Stone, the Triple Sound Stone. Sound from one end of the echo wall is continuously reflected by the wall repeatedly, to transmit to the other end. ^[4] If a person whispers to the wall at any point, his/her voice can be heard at any point along the wall. The Imperial Vault of Heaven (皇穹宇）is surrounded by the Echo Wall. Connecting the palace gate to the Imperial Vault of Heaven, there is a corridor made with 20 pieces of stone slabs. Standing on the first stone slab at the North end of the corridor and clap. The sound immediately spreads to the surrounding area through the air. Part of the sound is transmitted to one side of the Echo Wall, then it reflects and transmits the sound to the other side of the Echo Wall. The sound finally returns to the first slab as an echo, after this triangular-shaped transmission, people will be able to hear one echo. The first stone slab is named One Sound Stone. Clapping while standing on the second stone slab at the North end of the corridor, People will be able to hear two echoes. Part of the sound is transmitted to the Echo Wall, just like the first stone slab, which reflects and transmits the sound to the other side of the Echo Wall and sends back to the second stone slab as the first echo. The sound also sends to the East Annex Hall （东配殿）and the West Annex Hall （西配殿）and gets sent back to the stone slab as a second echo. Three echoes happen when standing on the third stone slab at the North end of the corridor. Clapping on that spot, different from others, the first echo comes from the East and West Annex Halls. The second echo is from the reflecting from the Echo Wall. The third echo comes from the second reflection wave from the Echo Wall. The uniqueness of the three echoes is that the volumes are all different from each other, the volume of the first echo is weak, the second is the loudest, while the third is the weakest. The time interval is also not the same. The first echo comes fast, as well as the second one, while the third echo takes longer to arrive. ^[5] MZ2160 (talk) 09:15, 1 December 2020 (UTC)Manlin Zhang MZ2160 (talk) 01:10, 2 December 2020 (UTC)Manlin Zhang MZ2160 (talk) 15:47, 2 December 2020 (UTC)Manlin Zhang