Acoustic attenuation and velocity measurements by an improved pulse-echo technique have been made in the critical region of methanol and cyclohexane critical mixture at its critical composition. The measurements have been performed in the frequency range 5-45 MHz and in the temperature range 34.3-57.9 degrees C. Large acoustic attenuation was observed in the entire temperature range. Fixman's theory (1962) of critical sound attenuation explains the experimental results reasonably well. The Debye short-range correlation lengths for this system are found to be 9.9*10-8 cm (above Tc) and 10.3*10-8 cm (below Tc).

Values of the nonlinearity constant “D,” which are instrumental in conversion of the acoustic phonons to thermal phonons, are determined in this paper for copper in the temperature range from 300° to 80°K, using experimental data of third-order elastic constants of this material. The previously predicted temperature-dependent behavior of this constant D, which found experimental support partially at room and liquid-nitrogen temperatures only in Ge, has been thus tested for the first time on experimental basis even for a metallic crystal. The results are found to be in agreement with the previous prediction. Moreover, the temperature-dependent curve for phonon viscosity component of hypersonic attenuation can be recalculated by using our values of D. When this is done, a better agreement is obtained than that obtained previously between the theoretical and corresponding experimental curves of hypersonic attenuation as a function of temperature for longitudinal wave propagation in the (100) direction.

Ultrasonic absorption and velocity measurements by an improved pulse-echo technique have been made in a critical mixture of water and phenol. The measurements have been made both above and below the critical temperature Tc in the frequency range 5-65 MHz. Large ultrasonic absorption was observed in both cases. The experimental results can be fairly well explained by Fixman's theory (1962) of critical acoustic attenuation in binary liquid mixtures. The Debye short-range correlation lengths for this system are found to be 5*10-8 cm (above Tc) and 5.14*10-8 cm (below Tc).

No abstract