Quasi-particle creation by analogue black holes

We discuss the issue of quasi-particle production by 'analogue black holes' with particular attention to the possibility of reproducing Hawking radiation in a laboratory. By constructing simple geometric acoustic models, we obtain a somewhat unexpected result: we show that in order to obtain a stationary and Planckian emission of quasi-particles, it is not necessary to create an ergoregion in the acoustic spacetime (corresponding to a supersonic regime in the flow). It is sufficient to set up a suitable dynamically changing flow. For instance, either a flow that eventually generates an arbitrarily small sonic region upsilon = c, but without any ergoregion, or even a flow that just asymptotically, in laboratory time, approaches a sonic regime with sufficient rapidity.