  In order to elucidate the enhancement of pion production that may occur
during dynamical scenarios of interest in connection with disoriented chiral
condensates, we study the evolution of boson modes whose frequencies have a
given arbitrary time dependence. The quantum-field treatment yields expressions
for the time dependence of observables that depend only on the initial state
and specific state-independent enhancement coefficients which can be obtained
from the given evolution of the frequency. It is shown how these coefficients
also can be obtained within an entirely classical framework by judicious usage
of the dependence of the resulting enhancement on the phase of the initial
state. Illustrative application is made for frequency evolutions obtained from
idealized simulations of high-energy heavy-ion collisions and it is shown how
the non-equilibrium evolution of the order parameter may lead to significant
enhancements in the final pion spectra.
