  Heavy ion collisions at RHIC are well described by the (nearly ideal)
hydrodynamics. In the present paper we study propagation of perturbations
induced by moving charges (jets) on top of the expanding fireball, using
hydrodynamics and (dual) magnetohydrodynamics. Two experimentally observed
structures, called a "cone" and a "hard ridge", have been discovered in
dihadron correlation function with large-$p_t$ trigger, while "soft ridge" is a
similar structure seen without hard trigger. All three can be viewed as traces
left by a moving charge in matter, on top of overall expansion. A puzzle is why
those perturbations are apparently rather well preserved at the time of the
fireball freezeout. We study two possible solutions to it: (i) a
"wave-splitting" acoustic option and (ii) a "metastable electric flux tubes".
In the first case we show that rapidly variable speed of sound under certain
conditions leads to secondary sound waves, which are at freezeout time closer
to the original location and have larger intensities than the first wave. In
the latter case we rely on (dual) magnetohydrodynamics, which also predicts two
cones or cylinders of the waves. We also briefly discuss metastable electric
flux tubes in the near-$T_c$ phase and their relation to clustering data.
