Sound propagation in 3He and 4He above the liquid-vapor critical point

A sealed plot is presented of published sound attenuation and dispersion data of (3)He and (4)He along the critical isochore above T(c) as a function of the reduced frequency omega*. Here omega* = omega/Gamma with omega the frequency and Gamma the order parameter relaxation rate, where the latter is determined from experimental transport and thermodynamic data. For a given isotope, the scaled data obtained at different frequencies lie on a single curve. These curves are different, however, for (3)He and (4)He. The resulting scaled plot is compared with recent predictions by Onuki, and by Folk and Mosel. The crossover-term contribution in Gamma from the background thermal conductivity data is compared with the form derived by Bhattacharjee and Ferrell. In the Appendix, the calculation of Gamma for both (3)He and (4)He from experiments is described and discussed.