Hydrodynamic sound experiments in normal and superfluidHe3

In a cylindrical resonator the attenuation and the velocity of first sound in normal and superfluid ${}_{}{}^3{}_{}{}^{}\mathrm{He}$ have been measured in the frequency range from 40 to 300 kHz and at pressures of 8, 18.8, and 28 bar. In the normal fluid the viscosity deduced from our data agrees with previously reported values; the sound velocity in this regime can be explained by taking into account mean-free-path effects and zero-sound corrections. The results for the viscosity in the superfluid confirm the previous observations of a continuous decrease down to the lowest temperatures in contradiction to theoretical predictions. The sound-velocity data in the superfluid regime indicate that specular reflection of the quasiparticles may occur at the resonator walls.