  Acoustic perturbations in a parallel relativistic flow of an inviscid fluid
are considered. The general expression for the frequency of the sound waves in
a uniformly (with zero shear) moving medium is derived. It is shown that
relativity evokes a difference in the frequencies of the sound-type
perturbations propagating along and against the current. Besides, it is shown
that the perturbations are not purely irrotational as they are in
nonrelativistic case. For a non-uniformly (with nonzero shear) moving fluid a
general set of equations, describing the evolution of the acoustic
perturbations in relativistic sheared flows, is obtained and analysed when the
temperature is nonrelativistic. It is shown that, like the nonrelativistic
case, in the new system: (a) the excitation of vortical, transiently growing
perturbations, and (b) the excitation of sound-type perturbations, extracting
the kinetic energy of the background flow, are possible. It is demonstrated
that the relativistic character of the motion significantly intensifies the
efficiency of both these processes. Finally, the possible relevance of this
study to processes in astrophysical shear flows is pointed out.
