Propagation of Collisionless Sound in Normal and Extraordinary Phases of LiquidHe3below 3 mK

Measurements of attenuation and velocity of sound in the collisionless regime have been made in normal and extraordinary phases of liquid ${}_{}{}^3{}_{}{}^{}\mathrm{He}$ at several pressures. The attenuation has a frequency-independent component with a sharp maximum just below $T_c$ and a frequency-independent component with a BCS-like temperature dependence. The velocity, frequency dependent very near $T_c$, becomes frequency independent at lower $T$ and may be approaching the velocity of first sound.