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The propagation velocity and the absorption coefficient of ultrasound in a frequency range of 5-2800 MHz in a n-pentanol-nitromethane solution in a vicinity of its critical stratification point from the homogeneous state side have been studied. The research make it possible to reveal the influence of concentration fluctuations on the sound propagation velocity. Three regions of dynamical parameters are analyzed: the mean-field (FL ≪ 1), fluctuation (FL ≫ 1), and transition (crossover, FL = 1) ones. On the basis of experimental data, the temperature dependence of the concentration fluctuation relaxation time () is studied, and its magnitude 0 is determined. The contribution to the fluctuation part of the sound absorption coefficient at high frequencies (> 300 MHz), which is connected with the sound scattering by concentration fluctuations near the critical stratification point is estimated.

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The results of the analysis of experimental data concerning the sound absorption in the nitromethane--pentanol and nitrobenzene--hexane solutions obtained in a wide frequency interval of 5-2800 MHz and measured along the isotherms and isoconcentrates, including their critical values, are presented. The detected anomalous dependences of the sound absorption coefficient were found to obey the laws of the dynamic scaling theory only in the fluctuation region of the problem parameters, fl ≫ 1. The sound frequency growth ( ≥ 110 MHz) in the examined frequency interval, as well as moving away from the critical temperature and concentration values, is proved to transit the system from the critical region into the crossover, fl ∼ 1, or even hydrodynamic, fl ≪ 1, one. K e y w o r d s: sound absorption coefficient, critical stratification point, binary solution, concentration fluctuations, acoustic relaxation.