Is anisotropic flow really acoustic?

The flow harmonics for charged hadrons ($v_{n}$) and their ratios $(v_n/v_2)_{n\geq 3}$, are studied for a broad range of transverse momenta ($p_T$) and centrality ($\text{cent}$) in Pb+Pb collisions at $\sqrt{s_{NN}}= 2.76$ TeV. They indicate characteristic scaling patterns for viscous damping consistent with the dispersion relation for sound propagation in the plasma produced in the collisions. These scaling properties are not only a unique signature for anisotropic expansion modulated by the specific shear viscosity ($\eta/s$), they provide essential constraints for the relaxation time, a distinction between two of the leading models for initial eccentricity, as well as an extracted $\left< \eta/s \right>$ value which is insensitive to the initial geometry model. These constraints could be important for a more precise determination of $\eta/s$.