  The scaling properties of the flow harmonics for charged hadrons $v_{n}$ and
their ratios $[ v_n/(v_2)^{n/2}]_{n\geq 3}$, are studied for a broad range of
transverse momenta ($p_T$) and centrality selections in Au+Au and Pb+Pb
collisions at $\sqrt{s_{NN}}=0.2 \text{and} 2.76$ TeV respectively. At
relatively low $p_T$, these scaling properties are found to be compatible with
the expected growth of viscous damping for sound propagation in the plasma
produced in these collisions. They also provide important constraints for
distinguishing between the two leading models of collision eccentricities, as
well as a route to constrain the relaxation time and make estimates for the
ratio of viscosity to entropy density $\eta/s$, and the "viscous horizon" or
length-scale which characterizes the highest harmonic which survives viscous
damping.
