  The present paper aims to complete an earlier paper where the acoustic world
was introduced. This is accomplished by analyzing the interactions which occur
between the inhomogeneities of the acoustic medium, which are induced by the
acoustic vibrations traveling in the medium. When a wave packet travels in a
medium, the medium becomes inhomogeneous. The spherical wave packet behaves
like an acoustic spherical lens for the acoustic plane waves. According to the
principle of causality, there is an interaction between the wave and plane wave
packet. In specific conditions the wave packet behaves as an acoustic black
hole.
