Effect of a change in Mach number onto the measured radio spectrum. Top panel: synchrotron-to-SZ flux ratio as a function of the shock Mach number, seen at different observing frequencies. In the DSA-based model this ratio varies rapidly only below ℳ ≲ 2.5, above which it asymptotes to a constant ratio that starts to decrease at high ℳ-values. For comparison, results from a simple ℳ³ dependence of the radio synchrotron power is shown with dotted lines, where the flux ratio increases linearly (see Sect. 4.2). Bottom panel: the observed radio spectra for different Mach number values, using the linear Mach dependence of the synchrotron-to-SZ flux ratio. The dashed lines are uncontaminated synchrotron spectra that follow a single power law throughout.