Characteristic strain spectrum of the injected GWB. Purple stars are the contributions to the spectrum of all the SMBHBs in the population. The black solid line is the total power spectrum in each frequency bin, where the frequency bins start at f = 1/T_obs and have width Δf = 1/T_obs with T_obs = 24.8 yr. As reference, the ideal spectrum with slope −2/3 and amplitude at the reference frequency of 1 yr⁻¹A_GW = 2.4 × 10⁻¹⁵ is shown by the dashed orange line.