  We investigate the formation of acoustic horizons for an inviscid fluid
moving in a pipe in the case of stationary and axi-symmetric flow. We show
that, differently from what is generally believed, the acoustic horizon forms
in correspondence of either a local minimum or maximum of the flux tube
cross-section. Similarly, the external potential is required to have either a
maximum or a minimum at the horizon, so that the external force has to vanish
there. Choosing a power-law equation of state for the fluid, $P\propto
\rho^{n}$, we solve the equations of the fluid dynamics and show that the two
possibilities are realized respectively for $n>-1$ and $n<-1$. These results
are extended also to the case of spherically symmetric flow.
