  The near-horizon metric for a black brane in Anti-de Sitter (AdS) space and
the metric near the AdS boundary both exhibit hydrodynamic behavior. We
demonstrate the equivalence of this pair of hydrodynamic systems for the sound
mode of a conformal theory. This is first established for Einstein's gravity,
but we then show how the sound damping constant will be modified, from its
Einstein form, for a generalized theory. The modified damping constant is
expressible as the ratio of a pair of gravitational couplings that are
indicative of the sound-channel class of gravitons. This ratio of couplings
differs from both that of the shear diffusion coefficient and the shear
viscosity to entropy ratio. Our analysis is mostly limited to conformal
theories but suggestions are made as to how this restriction might eventually
be lifted.
