Pulse-time-of-flight measurements have been used to determine the velocities of 10-MHz transverse and longitudinal sound in the hcp phase of solid helium-four at a molar volume of approximately 21 ${\mathrm{cm}}^3$ per mole. The measurements have been made on single crystals grown from superfluid helium. The orientations of these crystals have been determined by an optical birefringence technique. The five elastic constants for this molar volume of hcp solid helium have been calculated using the data obtained in these experiments.

• Received 7 August 1970

DOI:https://doi.org/10.1103/PhysRevA.3.1162