  In recent years, strong analogies have been established between the physics
of acoustic perturbations in an inhomogeneous dynamical fluid system, and some
kinematic features of space-time in general relativity. An effective metric,
referred to as the `acoustic metric', which describes the geometry of the
manifold in which acoustic perturbations propagate, can be constructed. This
effective geometry can capture the properties of curved space-time in general
relativity. Physical models constructed utilizing such analogies are called
`analogue gravity models'. Classical analogue gravity effect may be observed
when acoustic perturbations propagate through a inhomogeneous transonic
classical fluid. The acoustic horizon, which resembles a black hole event
horizon in many ways, is generated at the transonic point in the fluid flow.
The acoustic horizon is essentially a null hyper surface, generators of which
are the acoustic null geodesics, i.e. the phonons. The acoustic horizon emits
acoustic radiation with quasi thermal phonon spectra, which is analogous to the
actual Hawking radiation. The temperature of the radiation emitted from the
acoustic horizon is referred to as the analogue Hawking temperature.
  It has been demonstrated that, in general, the transonic accretion in
astrophysics can be considered as an example of the classical analogue gravity
model naturally found in the Universe.
