  Quasinormal ringing of acoustic black holes in Laval nozzles is discussed.
The equation for sounds in a transonic flow is written into a
Schr\"{o}dinger-type equation with a potential barrier, and the quasinormal
frequencies are calculated semianalytically. From the results of numerical
simulations, it is shown that the quasinormal modes are actually excited when
the transonic flow is formed or slightly perturbed, as well as in the real
black hole case. In an actual experiment, however, the purely-outgoing boundary
condition will not be satisfied at late times due to the wave reflection at the
end of the apparatus, and a late-time ringing will be expressed as a
superposition of "boxed" quasinormal modes. It is shown that the late-time
ringing damps more slowly than the ordinary quasinormal ringing, while its
central frequency is not greatly different from that of the ordinary one. Using
this fact, an efficient way for experimentally detecting the quasinormal
ringing of an acoustic black hole is discussed.
