Dependence of sound velocity in liquid helium-4 on temperature, pressure and frequency; a test of the phonon dispersion relation

The results of low frequency sound velocity measurements agree with phonon dispersion for small momenta of the form E=Cp(1+γp2...), with γ>0 at low pressures; γ decreases linearly with increasing density, reaching zero at a density corresponding to approximately 18 atm. (Auth.)