Sound Absorption in Critical Mixtures

Previous ultrasonic measurements in the binary mixtures aniline cyclohexane and nitrobenzene $n-\mathrm{hexane}$ have been analyzed according to a method recently proposed for Xe. The experimental results are compared with numerical calculations based on a theoretical formulation by Fixman and Kawasaki. At high frequency ($f^{*}>1\mathrm{}$), the behavior of the sound absorption is quite different from that observed in Xe. We find that the low-frequency form of the theory can be used to fit the data in the entire frequency range. In particular, the Kawasaki prediction ${\alpha }_{\lambda }\sim f^0{\epsilon }^0$ when $f^{*}\gg 1$, which seems to be confirmed by the experimental results in Xe, is not verified for the two mixtures examined here. We suggest that this different behavior is a consequence of the different behavior of the specific heats at constant volume.