Background contributions to sound waves at the liquid-heliumλtransition

The noncritical contributions to the sound near $T_{\lambda }\mathrm{}\left(P\right)\mathrm{}$ in ${}_{}{}^4{}_{}{}^{}\mathrm{He}$ liquid are analyzed quantitatively in terms of the dynamic scaling theory of Ferrell and Bhattacherjee. By using as background the thermodynamic velocity in a zero-height sample, an excellent agreement is found between the theoretical predictions and the experimental data for the dispersion of the velocity. In contrast, the observed background attenuation does not seem to be explained by a simply additive hydrodynamic contribution.