Integrated total intensity spectra of the relics considered in this paper. Top left: spectrum of the Sausage radio relic. The flux densities at frequencies lower than 4.85 GHz are taken from Table 3 of Stroe et al. (2016). Top right: spectrum of the Toothbrush radio relic. The flux densities at frequencies lower than 4.85 GHz are taken from Table 3 of Stroe et al. (2016). Bottom left: spectrum of the radio relic in ZwCl 0008+52. The flux densities at frequencies lower than 4.85 GHz are taken from van Weeren et al. (2011c). Bottom right: spectrum of the radio relic in Abell 1612. The flux densities at frequencies lower than 4.85 GHz are taken from van Weeren et al. (2011b).