  The motion of sound waves propagating in the perfect fluid with inhomogeneous
background flow is effectively described as a massless scalar field on a curved
space-time. This effective geometry is characterized by the acoustic metric,
which depends on the background flow, and null geodesics on the geometry
express the acoustic causal structure. Therefore by the effective geometry we
can easily study the causality on the flows. In this paper, we consider a
spherically symmetric, relativistic outflow and present the maximal causally
connected region for a super-sonic flow. When Lorentz factor of the radial
velocity of the flow is constant or obeys power-law with respect to the radial
coordinate r, we can solve it analytically. As a result we show that in the
constant case the maximum angle is proportional to inverse of Lorentz factor
and logarithmically increases with respect to r, in contrast, accelerative
expansions in power-law case make this angle bounded.
