Averaged, distance weighted line profiles. The SiO (2–1) line is shown in black, while the SiO (5–4) line is red. Dotted grey line indicates the velocity range Δv = 8 km s^-1 with respect to the averaged line centre. The panel a) corresponds to the H II regions; b) shows the infrared-bright sources; c) shows the infrared-quiet sample; and d) shows a subset of the infrared-quiet sample, where 1 <d< 7 kpc and M> 650M_⊙. The bottom panels show the ratio of the 5−4 over the 2−1 line intensity per 2 km s^-1 velocity bins.For a better comparison, a ratio of 0.75 is indicated with dotted grey line.