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We investigate the formation of acoustic horizons for an inviscid fluidmoving in a pipe in the case of stationary and axi-symmetric flow. We showthat, differently from what is generally believed, the acoustic horizon formsin correspondence of either a local minimum or maximum of the flux tubecross-section. Similarly, the external potential is required to have either amaximum or a minimum at the horizon, so that the external force has to vanishthere. 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 twopossibilities are realized respectively for $n>-1$ and $n<-1$. These resultsare extended also to the case of spherically symmetric flow.

Acoustic horizons for axially and spherically symmetric fluid flow