  Hydrodynamical description of the "Little Bang" in heavy ion collisions is
surprisingly successful: here we systematically study propagation of small
perturbations %, also treated hydrodynamically. Using analytic description of
the expanding fireball known as the "Gubser flow", we proceed to linearized
equations for perturbations. As all variables are separated and all equations
solved (semi)analytically, we can collect all the harmonics and reconstruct the
complete Green function of the problem, even in the viscous case. Applying it
to the power spectrum we found acoustic minimum at the $m=7$ and maximum at
$m=9$, which remarkably have some evidence for both in the data. We estimate
effective viscosity and size of the perturbation from a fit to power spectrum.
The shape of the two-point correlator is also reproduced remarcably well. At
the end we argue that independent perturbations are local, and thus harmonics
phases are correlated.
