  The study of acoustic black holes has been undertaken to provide new insights
about the role of high frequencies in black hole evaporation. Because of the
infinite gravitational redshift from the event horizon, Hawking quanta emerge
from configurations which possessed ultra high (trans-Planckian) frequencies.
Therefore Hawking radiation cannot be derived within the framework of a low
energy effective theory; and in all derivations there are some assumptions
concerning Planck scale physics. The analogy with condensed matter physics was
thus introduced to see if the asymptotic properties of the Hawking phonons
emitted by an acoustic black hole, namely stationarity and thermality, are
sensitive to the high frequency physics which stems from the granular character
of matter and which is governed by a non-linear dispersion relation. In 1995
Unruh showed that they are not sensitive in this respect, in spite of the fact
that phonon propagation near the (acoustic) horizon drastically differs from
that of photons. In 2000 the same analogy was used to establish the robustness
of the spectrum of primordial density fluctuations in inflationary models. This
analogy is currently stimulating research for experimenting Hawking radiation.
Finally it could also be a useful guide for going beyond the semi-classical
description of black hole evaporation.
