  Recent experiments demonstrating storage of optical pulses in acoustic
phonons based on stimulated Brillouin scattering raise a number of questions
about the spectral and temporal capacities of such protocols and the
limitations of the theoretical frameworks routinely used to describe them. In
this work, we consider the dynamics of photon-phonon scattering induced by
optical pulses with temporal widths comparable to the period of acoustic
oscillations. We revisit the widely adopted classical formalism of coupled
modes and demonstrate its breakdown. We propose a simple extension to
generalise the formulation and find potentially measurable consequences in the
dynamics of Brillouin experiments involving ultra-short pulses.
