(Top panel) Dependence of the ratio of the equivalent widths of the 12.7 and 11.2 μm PAH bands with the silicate strength. The dashed line indicates the average EW_12.7/EW_11.2 for the sample, while the dotted lines enclose the ±2σ interval. (Bottom panel) observed flux ratio between the two PAH bands as a function of S_sil. The dashed line represents a weighted linear fit to the sample, while the dotted lines mark a ±10 per cent deviation from this fit. Open squares, triangles, and circles highlight sources with unusual EW_12.7/EW_11.2, f_12.7/f_11.2, or both (see the text).