Title: CRITICAL SOUND IN FLUIDS AND MIXTURES
Authors: R.Folk (Institute for Theoretical Physics, University of Linz, Austria), G.Moser (Institute for Physics and Biophysics, University of Salzburg, Austria)

The behaviour of sound is explained within the dynamical renormalization group theory. Non-asymptotic effects are due to the deviation of renormalized couplings from their fixed point values. We calculate the temperature and frequency dependence of the sound velocity and absorption near the consolute point and the gas liquid critical point in pure fluids and mixtures. The critical non-asymptotic time scale in mixtures is different from the pure fluid case and set by an effective order parameter Onsager coefficient containing the dynamical parameter related to the enhancement of the thermal conductivity. We discuss the relation to the phenomenological theory of Ferrell and Bhattacharjee for the consolute point and compare with experiments in pure $^3$He, $^4$He and $^3$He-$^4$He mixtures near the plait point.

Comments: Figs. 4, Refs. 25, Tabs. 0.