  We consider an abstract linear wave equation with a time-dependent
dissipation that decays at infinity with the so-called scale invariant rate,
which represents the critical case. We do not assume that the coefficient of
the dissipation term is smooth, and we investigate the effect of its
oscillations on the decay rate of solutions.
  We prove a decay estimate that holds true regardless of the oscillations.
Then we show that oscillations that are too fast have no effect on the decay
rate, while oscillations that are in resonance with one of the frequencies of
the elastic part can alter the decay rate.
  In the proof we first reduce ourselves to estimating the decay of solutions
to a family of ordinary differential equations, then by using polar coordinates
we obtain explicit formulae for the energy decay of these solutions, so that in
the end the problem is reduced to the analysis of the asymptotic behavior of
suitable oscillating integrals.
