2026-07-072026-07-07http://salesiana.dossiersoluciones.com/handle/123456789/16318The average power spectrum of a pulse reflected by a disordered medium embedded in an N-mode waveguide decays in time with a power law t^(-p). We show that the exponent p increases from 3/2 to 2 after N^2 scattering times, due to the onset of localisation. We compare two methods to arrive at this result. The first method involves the analytic continuation to imaginary absorption rate of a static scattering problem. The second method involves the solution of a Fokker-Planck equation for the frequency dependent reflection matrix, by means of a mapping onto a problem in non-Hermitian quantum mechanics.4 pages, 1 figure, reorganized versionDisordered Systems and Neural NetworksMesoscale and Nanoscale Physicstext