Power spectral densities in the 0.5–10 keV band of three representative observations of Cyg X-1 associated with the hard (blue) and soft spectral state (red). Observation 0100320110 at Γ ∼ 2.3 is in the transition region between these states (green). The hard state can be characterized by strong broad band noise that can be resolved into multiple Lorentzian components (blue dashed lines). The two most prominent Lorentzians, L₁ and L₂, are labeled. In the transition, Cyg X-1 exhibits roughly a single broad component. The soft state shows almost featureless red noise.