Distribution of the amplitudes and frequencies of (independent) signals not identified as combination frequencies for all 38 SPB stars in our sample, for each of the pre-whitening strategies defined in Table 1. The colour indicates if these signals are involved in a candidate resonance (red) or not (blue), and its gradient indicates the number of detected frequencies: Colours are brighter if more signals are detected within a specific amplitude-frequency bin. The red and blue distributions are plotted independently, so overlap between these distributions is the cause for the appearance of darker, maroon-like coloured bins belonging to the red distribution. The maximal frequency displayed in each panel is 8 d⁻¹, which is the highest frequency of any of the detected (robust two-signal) candidate resonances (see Sect. 4 and, especially, Fig. 10). We display independent signals with an amplitude > 10 ppm because very few independent signals have amplitudes smaller than this limit.