Frequency dependence of the amplitudes A^EE,BB of the angular power spectra, relative to 353 GHz (see details in Sect. 4.5). Results for (red squares) and (blue circles) for the smallest region, LR24. These include evaluations from cross-spectra involving polarization data at two frequencies, plotted at the geometric mean frequency. The square of the adopted relative SED for dust polarization, which is a modified blackbody spectrum with β_d = 1.59 and T_d = 19.6 K, is shown as a black dashed line. The ± 1σ uncertainty area from the expected dispersion of β_d, 0.03 for the size of LR24 as inferred from Planck Collaboration Int. XXII (2015) (see Sect. 2.2.1), is shown in grey.