  The fact that fast oscillating homogeneous scalar fields behave as perfect
fluids in average and their intrinsic isotropy have made these models very
fruitful in cosmology. In this work we will analyse the perturbations dynamics
in these theories assuming general power law potentials $V(\phi)=\lambda
\vert\phi\vert^{n}/n$. At leading order in the wavenumber expansion, a simple
expression for the effective sound speed of perturbations is obtained
$c_{\text{eff}}^2 = \omega=(n-2)/(n+2)$ with $\omega$ the effective equation of
state. We also obtain the first order correction in
$k^2/\omega_{\text{eff}}^2$, when the wavenumber $k$ of the perturbations is
much smaller than the background oscillation frequency, $\omega_{\text{eff}}$.
For the standard massive case we have also analysed general anharmonic
contributions to the effective sound speed. These results are reached through a
perturbed version of the generalized virial theorem and also studying the exact
system both in the super-Hubble limit, deriving the natural ansatz for
$\delta\phi$; and for sub-Hubble modes, exploiting Floquet's theorem.
