Sound Propagation in the Binary System Aniline-Cyclohexane in the Critical Region

We report here measurements of the absorption coefficient and velocity of ultrasound in the critical liquid-mixture aniline-cyclohexane ($T_c=29.8\mathrm{}°$C) as a function of frequency and temperature. The standard pulse technique was used for the absorption measurements over the frequency range between 1 and 75 Mc/sec. In this range, measurements have been performed at a minimum of 8 and a maximum of 11 frequencies. The temperature range covered is between 30.5 and 36.2°C. Velocity measurements have been performed in the same temperature range but only at 2 and 15 Mc/sec. The experimental data have been analyzed in terms of Fixman's theory. We also include a discussion of this theory, together with a comparison with the more recent works by Romanov and Solov'ev, Kawasaki and Tanaka, and Kadanoff and Swift. Our results are not conclusive, but we have provided further indications that the sound-diffusion coupling, originally suggested by Fixman, is the only mechanism responsible for the excess absorption in a region not too close to the critical one. Some limitation appears for the temperature closest to the critical point.