  We consider a massless scalar field in 1+1 dimensions inside a cavity
composed by a fixed plate, which imposes on the field a Robin BC, and an
oscillating one, which imposes on the field a Dirichlet BC. Assuming that the
plate moves for a finite time interval, and considering parametric resonance,
we compute the total number of created particles inside the cavity. We
generalize our results to the case of two parallel plates in 3+1 dimensions.
