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In this paper, we study the relations between phonetic features and acoustical signals. Because of the periodicity characteristic of voiced sounds, a period of a signal may be expanded in the Fourier series. A hypothesis that the characteristics of a simple vowel depend only on a period T and the coefficientsof a period of the simple vowel, where ai and bi are Fourier coefficients, is proposed. The characteristics of a diphthong depend on data of two simple vowels. The hypothesis is verified by synthesis and applied to recognition of vowels. Experiments are done for Mandarin syllables. A rule of changing tones and the characteristic of a stress sound are also provided in this study.