The assumption of the relaxation of the reaction MgSO4←→Mg+++SO4-- to be the basis of sound absorption in water has not sufficient ground. In this paper, the author derives a formula for relaxation absorption based on the chemical thermodynamics, and the supposition from which the phenomenological theory started is also explained. With the consideration of Debye-Hückel ionic atmosphere theory, the formula obtained is applied to the water solution of MgSO4. A critical discussion is given to the existing theories.