Since the Qing Dynasty, many people have participated in the study of the tone system of ancient Chinese. Gu put forward in "On Sound": "Although the theory of four tones originated in Jiangzuo, ancient poems have their own differences; Therefore, some people are flat and rhythmic, while others are not. If they go up or turn to Ping, go or turn to Ping, enter or turn to Ping, they will compete with singers. So four tones can be used together. " In other words, although the ancients had four tones, each word was not limited to reading a certain tone, and it could be changed as needed. The so-called "four tones are consistent."
According to Duan Yucai's Preface to Seventeen Chapters of the Book of Songs, "There are only three tones in Zhou and Qin Dynasties, namely, entering the tone quietly and being silent. It was not until the Wei and Jin Dynasties that disyllabic words gradually appeared, and they entered the tone from the upper voice. Qin Zhou's flat voice gradually turned into a deep voice. He also said that in ancient times, even tone was a kind of sound, with the same tone, similar tone and similar tone, and similar tone "(Wang Li's Chinese Phonology).
Kong Guangsen said in the preface to poetry: "As for entering the voice, it should be divided into seven parts, and besides closing the voice, it should be turned into silencing. Gai Rusheng was born in Jiangzuo, not the Central Plains. " That is to say, there was no sound in ancient times
Later, Huang Kan thought that ancient times were supreme and went to the second sound; Wang Li believes that there were two sounds in ancient times, namely "Shu" and "Confucianism". Shu has "long Shu" and "long Shu", while Confucianism has "long Shu" and "short Confucianism" In the Middle Ages, the long Shu became a flat tone, the short Shu became an ascending tone and a falling tone, and the short Shu entered the tone in the Middle Ages. Dong Tonghe thinks that the tone of ancient times is not much different from that of the Middle Ages.