  We propose a physical framework for interpreting the characteristic
frequencies seen in the broad band power spectra from black hole and neutron
star binaries. We use the truncated disc/hot inner flow geometry, and assume
that the hot flow is generically turbulent. Each radius in the hot flow
produces fluctuations, and we further assume that these are damped on the
viscous frequency. Integrating over radii gives broad band continuum noise
power between low and high frequency breaks which are set by the viscous
timescale at the outer and inner edge of the hot flow, respectively.
Lense-Thirring (vertical) precession of the entire hot flow superimposes the
low frequency QPO on this continuum power.
  We test this model on the power spectra seen in the neutron star systems
(atolls) as these have the key advantage that the (upper) kHz QPO most likely
independently tracks the truncation radius. These show that this model can give
a consistent solution, with the truncation radius decreasing from 20-8 Rg while
the inner radius of the flow remains approximately constant at ~4.5 Rg i.e. 9.2
km. We use this very constrained geometry to predict the low frequency QPO from
Lense-Thirring precession of the entire hot flow from r_o to r_i. The simplest
assumption of a constant surface density in the hot flow matches the observed
QPO frequency to within 25 per cent. This match can be made even better by
considering that the surface density should become increasingly centrally
concentrated as the flow collapses into an optically thick boundary layer
during the spectral transition. The success of the model opens up the way to
use the broad band power spectra as a diagnostic of accretion flows in strong
gravity.
