The Echo Wall, which surrounds the Imperial Vault of Heaven courtyard, is so named because of the way it produces echoes. Built of refined dengjiang bricks from Linqing that were joined with extreme precision, the wall is incredibly smooth with a highly precise curvature, making it such a good reflector of sound waves. Sound loses barely any of its volume upon being reflected by the wall and is continuously reflected as it travels along the wall, creating the effect of one’s voice sounding close even when the listener is far away.