Upper panel: amplitude of computed power spectra on data for different multipole bins and sky masks (the color scheme is the same of Fig. 4) as a function of the effective frequency. Filled points refers to EE spectra, empty ones to BB. Curves represent the best model we obtain when fitting Eq. (2) to data (solid and dashed lines for E and B-modes respectively). We note that in this figure the amplitude of D_ℓ = ℓ(ℓ +1)C_ℓ/2π is plotted. Lower panel: residuals of the fits normalized to the 1σ error, the dashed horizontal lines represent the 2σ level. In both upper and lower panel the points inside the gray shaded area come from the correlation of S-PASS with WMAP/Planck data which, as described in the text, are not considered in the fitting.