Measurements of the longitudinal sound velocity $v_1$ in hcp ${\mathrm{He}}^4$ as a function of direction relative to the hexagonal axis are reported for molar volumes between 20.3 and 17.3 ${\mathrm{cm}}^3$/mole. The variation of $v_1$ with molar volume is much stronger than predicted by theory. Agreement is found with theoretical predictions for the anisotropy of $v_1$.

• Received 17 December 1969

DOI:https://doi.org/10.1103/PhysRevLett.24.365