A number of investigators have discussed the use of two-detector cross power spectral density (CPSD) measurements to obtain the velocity of an axially propagating perturbation of the moderator density in a boiling water reactor. The basis of the measurements is the view that the detector response can be separated into local and global components where the local component, which is dominant for high frequencies (f > 2 Hz), makes possible the observation of the moving perturbation associated with steam flow in the reactor. For low frequencies, the response consists of a combination of the local and global components, and correlation with the perturbation velocity is not straightforward. In this Note, the asymptotic low and high frequency behavior of the CPSD is examined using the complex detector adjoint function formulation. It is shown that at low frequencies, where the wavelength of the perturbation is much larger than the axial core dimension, the phase of the CPSD and therefore the perturbation velocity correlates with the centroid spacing of detector functions involving the product of the detector adjoint function and the static flux. For high frequencies, on the other hand, the phase correlates with the detector spacing. This behavior is considered to be an alternate manifestation of the local/global concept. Numerical calculations based on a two-group, one-dimensional model are used to illustrate these observations. It is also shown using the model that the oscillations in the phase in the intermediate frequency range disappear for frequencies that correspond to wavelengths that are intergral multiples of the core height.