Propagation of elastic waves in granular solid hydrodynamics.

The anisotropic stress-dependent velocity of elastic waves in glass beads--as observed by Khidas and Jia [Phys. Rev. E 81, 021303 (2010)]--is shown to be well accounted for by "granular solid hydrodynamics," a broad-range macroscopic theory of granular behavior. As the theory makes no reference to fabric anisotropy, the influence of which on sound is in doubt.