Mean GW energy density spectrum from massive BH mergers compared with the sensitivities of different experiments. The black dashed curve shows the case where all mergers of galaxies produce a GW signal for BHs heavier than 10³ M_⊙ (p_BH = 1). The colored bands show the spectra from SMBHs heavier than 10⁹ M_⊙ (dark blue), from SMBHs in the range (10⁶ M_⊙, 10⁹ M_⊙); light blue), and from IMBHs with masses in the range 10³ − 10⁶ M_⊙ (green), assuming a universal efficiency factor . The shaded regions show the prospective sensitivities of IPTA30 (Chen et al. 2017; Kaiser & McWilliams 2021), LISA (LISA Collaboration 2017), AEDGE (El-Neaj et al. 2020; Badurina et al. 2021), AION-km (Badurina et al. 2020, 2021) and ET (Sathyaprakash et al. 2012).