The phase-velocity spectrum of background internal friction has been shown to be a material property. The shape of such spectra seems to be different, in particular, for monophasic (e.g. pure metals) and multiphase materials (typically composites and rocks). An explanation for such difference. as yet not known, is proposed in the present work. All spectra are interpreted to be a combination of two or more simple relaxation sources, each emanating from point defect diffusion with a specific diffusion scale length and inducing dissipation into one another. The single dissipation sources are assumed to combine with the laws of linear circuits in parallel. In the case of monophasic metals a model with two sources of dissipation, one in the microstructure and the other in the lattice, fits the experimental spectra qualitatively well. For multiphase materials, including natural rocks and artificial conglomerates, which exhibit flat spectra, a continuous-source model is suggested. (C) 2002 Elsevier Science B.V. All rights reserved.