Characteristic frequency, v_* as a function of the Galactic longitude for SNRs with low frequency turnovers (filled symbols) and SNRs spectra without turnovers (open symbols). Circles correspond to Castelletti et al. (2021) plus this work data, while squares to Kovalenko et al. (1994b), and triangles to Kassim (1989). The solid line corresponds to the median prediction of the model presented in Fig. 9, which does not show a strong dependence with the Galactic longitude.