《汉书·志·律历志上》
声音，就是以宫为中间，从角慢慢开始，到征逐渐强大，而到商就更显著了，到羽就扩大到了整个宇宙，所以四声成为五音了。
夫声者，中于宫，触于角，祉于徵，章于商，宇于羽，故四声为宫纪也。
The sound is with the gong as the center, starting slowly from jiao, gradually becoming stronger until zheng, and becoming even more prominent until shang, finally reaching yu it expands to the entire universe, so the four sounds become five notes.