Properties of the observed spectra. The observations are plotted in black and gray dots for the observations with and without detected 69 μm bands respectively. In panel A) is plotted the peak-over-continuum strength of the 69 μm band versus the F_30μm/F_18μm color of the spectrum. In panel B) the strength of the 11.3 μm band is plotted against the width of the 69 μm band. In panel c) the peak-over-continuum strength of the 69 μm band and the strength of the 11.3 μm band are shown. The peak-over-continuum strength of the 69 is plotted against the width of the 69 μm band in panel D).