  We study gravitational wave (GW) emission in the strong-field regime by a
hierarchical triple system composed of a binary system placed in the vicinity
of a supermassive black hole (SMBH). The LIGO-Virgo collaboration recently
reported evidence for coalescences with this dynamical origin. These systems
are common in galactic centers and thus are a target for the space-based LISA
mission as well as other advanced detectors. Doppler shifts, aberration,
lensing and strong amplitude modulations are features present in the GW signal
from these systems, built into our framework and with no need for
phenomenological patches. We find that the binary can resonantly excite the
quasinormal modes of the SMBH, as in the resonant excitation of two tuning
forks with matching frequencies. The flux of energy crossing the SMBH horizon
can be significant, when compared with that from standard extreme-mass-ratio
inspirals. Therefore, these triple systems are an outstanding probe of
strong-field physics and of the BH nature of compact objects.
