【解析】只写IPA 的定义是不够的，还需要加上别的东西让答案更加饱满，比如变音符。
IPA is a standardized and internationally accepted system of phonetic transcription. Its basic principle is using a different letter for each distinguishable speech sounds. With minor modification it is now still used by phoneticians and linguists.
Diacritics：As some speech sounds produced differ only in some detailed aspects, the IPA provides its users with another set of symbols called diacritics, which are added to the letter-symbols to bring out the finer distinctions than the letters alone can possibly do. For example, [ ph ] (as in peak)，[p] (as in speak).
