Radio flux density, normalised by the characteristic flux and power-law emission, plotted against normalised frequency for the four subsamples in our study (see labels). Each spectrum was fitted by the respective authors with an exponential drop-off with an exponent −2.1. The curved solid line represents the normalised absorption function A(x) = exp(−x^−2.1), and the asterisk indicates the characteristic frequency, ν_∗.