Note that a black hole of physical mass M at a redshift of z will appear as a black hole of mass Mz = (1+ z)M. This shifts the frequency of the QNM to the lower end of the spectrum. Assuming a frequency cutoff of 10–4 Hz for LISA, this means that only black holes of intrinsic mass M < 1.2× 108M ⊙∕(1+ z) can be observed at a redshift z.