  We present a combined experimental and theoretical study of the drag force
acting on a high porosity aerogel immersed in liquid ${}^3$He and its effect on
sound propagation. The drag force is characterized by the
  Knudsen number, which is defined as the ratio of the quasiparticle mean free
path to the radius of an aerogel strand. Evidence of the Knudsen-hydrodynamic
crossover is clearly demonstrated by a drastic change in the temperature
dependence of ultrasound attenuation in 98% porosity aerogel. Our theoretical
analysis shows that the frictional sound damping caused by the drag force is
governed by distinct laws in the two regimes, providing excellent agreement
with the experimental observation.
