  We analyze the power spectra of the lightcurves of long gamma-ray bursts,
dividing the sample in bins of luminosity, using the recently discovered
variability-luminosity correlation. We find that the value of the variability
parameter strongly correlates with the frequency that contains most of the
power in the burst comoving frame. We compute the average power spectra in
luminosity bins. The average power spectrum is well described by a broken
power-low and the break frequency is a function of the variability parameter,
while the two slopes are roughly constant. This allow us to conclude that
scattering processes do not play a relevant role in modelling the lightcurves.
We finally discuss in which conditions scattering may still play a relevant
role in shaping the spectra of GRBs.
