  The universal behaviour of the magnetic excitations in high-temperature
superconductors is described in a model with static stripes retaining only the
localized spin degrees of freedom. The stripes are represented by a model of
coupled two-leg spin ladders. We start from the results obtained previously by
continuous unitary transformations for an isolated spin ladder. A quantitative
description of neutron scattering data is reached, using a model for a single
cuprate layer with well established values of the exchange coupling constants.
The neutron resonance peak is explained in terms of a saddle point in the
dispersion of the magnetic excitations. Here we make predictions for bilayer
systems with in-phase or out-of-phase stripe correlations. The results may
serve as a guide for future experimental analyses.
