  We develop the power spectral density (PSD) model to explain the nature of
the X-ray variability in IRAS 13224-3809, including the full effects of the
X-ray reverberation due to the lamp-post source. We utilize 16 XMM-Newton
observations individually as well as group them into three different luminosity
bins: low, medium and high. The soft (0.3-1 keV) and hard (1.2-5 keV) PSD
spectra are extracted and simultaneously fitted with the model. We find that
the corona height changes from h ~ 3 $\ r_{\rm g}$ during the lowest luminosity
state to ~ $25 \ r_{\rm g}$ during the highest luminosity state. This provides
further evidence that the source height from the reverberation data is
significantly larger than what constrained by the spectral analysis.
Furthermore, as the corona height increases, the energy spectrum tends to be
softer while the observed fractional excess variance, $F_{\rm var}$, reduces.
We find that the PSD normalization is strongly correlated with $F_{\rm var}$,
and moderately correlated with the PSD bending index. Therefore, the
normalization is dependent on accretion rate that controls the intrinsic shape
of the PSD. While the intrinsic variability of the disk is manifested by the
reverberation signals, the disk and corona may evolve independently. Our
results suggest that, during the source height increases, the disk itself
generates less overall variability power but more high-frequency variability
resulting in the PSD spectrum that flattens out (i.e. the inner disk becomes
more active). Using the luminosity-bin data, the hint of Lorentzian component
is seen, with the peak appearing at lower frequencies with increasing
luminosity.
