  The new formal analogy between superfluid systems and cosmology, which
emerges by taking into account the back-reaction of the vacuum to the quanta of
sound waves \cite{noi}, enables us to put forward some common features between
these two different areas of physics. We find the condition that allows us to
justify a General Relativity (GR) derivation of the hydrodynamical equation for
the superfluid in a four-dimensional space whose metric is the Unruh one
\cite{Unruh}. Furthermore we show how, in the particular case taken into
account, our hydrodynamical equation can be deduced within a four-dimensional
space from the wave equation of a massless scalar field.
