《韩非子·外储说右上》
对那些能在转音之后回复到清越微音的，才加以教授。另一种说法：教歌的人先用音法测验，要求学唱的人急呼合于宫调，慢呼合于微调。
一曰：教歌者先揆以法，疾呼中宫，徐呼中徵。
As for the student who can change from a high note to a clear and soft note, he gets lessons from the music teacher. Another theory is: The music teacher uses the music method to examine the student, asking the student to call out quickly to match the gongdiao and call out slowly to match the weidiao.