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This paper mainly describes the detection by the Infrasonic receiving array of the Sound-gravity wave produced by the eruption of the St. Helens volcano in the U.S.A. on May 18, 1980. The wave train chart of the sound pressure and the group velocity of the Infra-Sonic Signals, are given. Analysis of the dynamic spectrum has been carried out. The energy of this volcanic eruption is estimated, and the result is 30.6 MT.