One problem is that supermassive BHs merge at very low frequencies; they merge before their orbits get fast. LIGO's sensitivity range is in about the same rang

One problem is that supermassive BHs merge at very low frequencies; they merge before their orbits get fast. LIGO's sensitivity range is in about the same range of frequencies as the human ear. NSNS mergers reach our sensitivity sweet spot for a few seconds; intermediate mass BHBH mergers get into the low end of our sweet spot. Supermassive black holes merge at very low frequencies outside of our sensitive band. LISA is a planned space-based observatory that would be sensitive in those bands: https://lisa.nasa.gov/ https://lisa.nasa.gov/