30 pages, 16 figures.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 dynamically changing flow either eventually generating an arbitrarily small sonic region v=c, but without any ergoregion, or even just asymptotically, in laboratory time, approaching a sonic regime with sufficient rapidity.C.B. has been funded by the spanish MEC under project FIS2005-05736-C03-01 with a partial FEDER contribution. C.B. and S.L. are also supported by a INFN-MEC collaboration. The research of M.V. was funded in part by the Marsden Fund administered by the Royal Society of New Zealand.Peer reviewed