The authors show how a recent method, aimed at extracting the large-scale spectrum of cosmic inhomogeneities from the cosmic-background anisotropies without making a priori assumptions on its shape, can be extended to linear perturbations of any kind in cosmologies with arbitrary density parameter and cosmological constant. Applying the method to density waves in open models, from COBE-DMR's data one finds steep spectra at low cosmic densities, eg n= 1.91±0.47 for Ω 0= 0.3.
