The phonetic change of language flow is a temporary change. In verbal communication, phonetic changes can be seen everywhere. The same phonetic structural unit will change differently because of different phonetic environments; Even if the phonetic environment is the same, different people will have different pronunciations, that is, the same person will have different pronunciations at different times. Therefore, when we study, we can only sum up the common and universal sound change phenomenon in the language flow.
Historical phonetic changes refer to the phonetic changes of a specific language from one era to another. It is fixed and diachronic. For example, the changes of Chinese phonetic system from ancient times to modern times.
Other changes
Assimilation: two different phonemes, one of which is influenced by the other and becomes the same or similar phoneme. For example, the plural S in English, the unvoiced S after voiced consonants, such as books, and the Z after voiced consonants, such as dogs.
Alienation: two phonemes are the same and similar, and one of them becomes different from it for some reason. Tone sandhi in Putonghua is a typical alienation phenomenon. For example, the upper voice, the two upper voices are connected, and the former upper voice becomes a rising tone, such as "Tiger, Exhibition".
Weakening: In the language flow, the pronunciation of some phonemes becomes weak and unclear, which is called weakening. The light tone of Putonghua is a typical weakening phenomenon.
Falling off: the weakening is further strengthened, some phonemes are lost, or some reasons are missed in the process of rapid pronunciation, resulting in falling off. Say "I am" as "I am" in English