Introduction

Black holes are the vacuum solutions of Einstein's field equations in general relativity. Classically, a black hole is conceived as a singularity in space time, censored from the rest of the Universe by a mathematically defined one way surface, the event horizon. Inspite of the remarkable resemblance in between a black hole and an ordinary thermodynamic system, black holes never radiate according to the classical laws of physics. The introduction of quantum effects radically changes the scenario. Black holes radiate due to quantum effects. Such radiation is known as Hawking radiation and the corresponding radiation temperature is referred as the Hawking temperature. Observational manifestation of Hawking effect for astrophysical black holes is beyond the scope of present day's experimental techniques. Also, Hawking quanta may posses trans-Planckian frequencies, and physics beyond the Planck scale is not well understood. The above mentioned difficulties with Hawking effect were the motivations to search for an analogous version of Hawking radiation, and the theory of acoustic/analogue black holes were thus introduced.

Classical black hole analogue (alternatively, the analogue systems) are fluid dynamical analogue of general relativistic black holes. Such analogue effects may be observed when the acoustic perturbation (or equivalent perturbation, a surface gravity wave for example) propagates through a classical dissipation-less transonic fluid. The acoustic horizon, which resembles the actual black hole event horizon in many ways, may be generated at the transonic point in the fluid flow. Acoustic horizon emits acoustic radiation with quasi thermal phonon spectra, which is analogous to the actual Hawking radiation.