Sonic Mach numbers of turbulent motions in the 1°-wide squares chosen along the Eridu (green) and Reticulum (magenta) filaments, in the fibre (top) and shell (bottom) geometries, assuming isotropic turbulence and using the S_v dispersion function (dark filled symbols) or the σ_v line width (light open symbols) to trace the one-dimensional turbulent velocity dispersion. The data points and error bars respectively give the mean value in each ~5 pc-wide square and the rms dispersion at 0.5–0.7 pc scale inside each square. The histogram shows the Mach number distribution in cold CNM clumps from Heiles & Troland (2003).