Abstract The authors comprise the description of the noise phenomena taking place in a multivariable coupled reactor system into a comprehensive space dependent model. Using matrix methods a computer code was elaborated for calculating the spectra of the appropriately chosen state variables. The method is based on reactor data and coupling constants taken from static neutronphysical and thermalhydraulic codes. To solve the basic dynamical equations the frequency dependent eigenvalues of the system matrix are evaluated. These quantities have proved to be very useful in interpreting the low frequency behaviour of the spectra and in the study of local, global and mixed spatial effects with special respect to coupling and feedback processes in PWRs.