Left panel: 90% conservative upper limits on Mach number versus temperature. Point size refers to the physical scale and the color is coded according to the central entropy, K₀, in units of keV cm^-2. The lines show the upper limits on Mach number averaged in the four ranges of physical scales to underline its dependence on the source distance. Right panel: same as before, but here the limits on Mach number are scaled by the 1/3rd power of the physical scale for a comparison between sources at different redshift (assuming Kolmogorov turbulence, see Sect. 5.2).