What is "eight tones"?

As early as the Pre-Qin Dynasty, traditional Chinese musical instruments had a classification method based on the materials used to make them, which was called "Bayin" at that time. For example, bells, yongs, gongs, bells, gongs, etc. are made of metal and are classified as "gold"; chimes and chimes are made of stone and are classified as "stone"; xuns, fou, etc. are made of clay and are classified as The "earth" category; harps, leather drums, etc. use animal leather as the eardrum for percussion, and are classified as "leather"; qin, zither, zither, pipa, huqin, etc. use silk threads as strings for playing, and are classified as "silk" "category; ,, clappers, wooden clappers, wooden fish, etc. are made of wood and are classified as "wood"; Sheng, Lusheng, etc. are made of gourds or wooden gourds for pronunciation, and are classified as "匏" category; Chi, Xiao, flute, etc. are made of bamboo tubes and are classified into the "bamboo" category. In this way, "gold, stone, soil, leather, silk, wood, gourd and bamboo" are divided into eight categories, so they are called "eight tones".