Interferometric flux measurements of the Sausage relic in CIZA J2248.8+5301 (top panel) and the Toothbrush relic in 1RXS J0603.3+4214 (bottom panel) against our spectral predictions for three different shock Mach numbers. The data are taken from Stroe et al. (2016). The predicted spectra are normalized to have minimum χ² computed from the low-frequency (≲ 2 GHz) part. These model spectra correspond to an “ideal” measurement, and the results from actual interferometric imaging can differ (see text).