  Recent studies have shown that fluctuations of various types play important
roles in the evolution of the fireball created in relativistic heavy ion
collisions and bear many phenomenological consequences for experimental
observables. The bulk dynamics of the fireball is well described by
relativistic hydrodynamic expansion and the fluctuations on top of such
expanding background can be studied within the linearized hydrodynamic
framework. In this work we present complete and analytic sound wave solutions
on top of both Bjorken flow and Hubble flow backgrounds. We also briefly
discuss the applications of these solutions in heavy ion collisions.
