Sound waves in the intracluster medium

We present an analysis of the behaviour of a perturbed radio cocoon. Comparisons with observations of sound waves detected in the Perseus and Virgo clusters suggest the separations of observed ripples correspond to the natural oscillation frequency of the cocoon. An energy injection rate consistent with active galactic nucleus power is required to offset the strong acoustic damping of cocoon oscillations, suggesting the sources are in equilibrium with the intracluster medium (ICM), and the oscillations are effectively undamped. Viscous dissipation of sound waves provides ICM heating that can quench cooling flows on time-scales greatly exceeding the oscillation time-scale. Thermal conductivity is likely to be heavily suppressed.