Recent experiments on the attenuation and velocity of sound in liquid He⁴ at low temperatures are discussed in terms of the excitation model of liquid He⁴. By assuming for the long-wavelength excitations the dispersion relation ε(p)=cp[1-γp²-δp⁴⋯] with γ negative, we are able to reconcile previous disagreements between theory and experiment.
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Maris et al. (1970) studied this question.