Gravity effects on first-sound velocity nearTλ(P) in liquidHe4

Near $T_{\lambda }$, the first-sound velocity is mainly affected by two rounding mechanisms: one, intrinsic, is due to critical dispersion; the other, nonintrinsic, is due to the spatial inhomogeneity induced by gravity in a sample of finite height. Here a detailed analysis of the local and nonlocal gravity affects is made, and a numerical procedure to calculate quantitatively the gravity rounding contribution in the presence of dispersion is proposed. This method is used to separate for what we believe to be the first time gravity and dispersion effects in real measurements. In particular, contrary to a widely held assumption, it turns out that gravity effects appreciably the absolute value of the measured velocity only in the very-low-frequency range (a few kHz or less), independently of the sample height and of how close the system is to $T_{\lambda }$(P). It is also shown that the other main gravity effect, the temperature shift of the velocity minimum, is also strongly affected by dispersion. The present numerical procedure permits gravity corrections to (first-) sound velocity in real measurements to be made in a self-consistent way, with high accuracy.