Median GWB spectra produced by a subset of the new-physics models, which we construct by mapping our model parameter posterior distributions to h 2ΩGW distributions at every frequency f (see Appendix B for more details and Figures 19 and 20 for the models not included here). We also show the periodogram for an HD-correlated free spectral process (gray violins) and the GWB spectrum produced by an astrophysical population of inspiraling SMBHBs with the parameters A BHB and γ BHB fixed at the central values μ BHB of the 2D Gaussian prior distribution specified in Equation (A1) (black dashed line).