  Quantum and thermal fluctuations of an irrotational fluid are studied across
the transition regime connecting a protoinflationary phase of decelerated
expansion to an accelerated epoch driven by a single inflaton field. The
protoinflationary inhomogeneities are suppressed when the transition to the
slow roll phase occurs sharply over space-like hypersurfaces of constant energy
density. If the transition is delayed, the interaction of the quasi-normal
modes related, asymptotically, to fluid phonons and inflaton quanta leads to an
enhancement of curvature perturbations. It is shown that the dynamics of the
fluctuations across the protoinflationary boundaries is determined by the
monotonicity properties of the pump fields controlling the energy transfer
between the background geometry and the quasi-normal modes of the fluctuations.
After corroborating the analytical arguments with explicit numerical examples,
general lessons are drawn on the classification of the protoinflationary
transition.
