Standard deviation of the mean-normalized column densities, , as a function of the sonic Mach number, ℳ_s, in eight IRDCS. The Cloud E is not included in the plot, because of the potential problems in the extinction data of the cloud (see Sect. 2.4). The Cloud I is also excluded because of nebulosity in the cloud region. The left frame shows the relation when ℳ_s is calculated from Gaussian fits to the mean spectra. The right frame shows the relation when ℳ_s is calculated from a fixed velocity range (see Sect. 2.4). The A_V = 7 mag box was to define the cloud region that was included in the plots (see Sect. 3.2).