  Out of equilibrium quasiparticles (QPs) are one of the main sources of
decoherence in superconducting quantum circuits, and are particularly
detrimental in devices with high kinetic inductance, such as high impedance
resonators, qubits, and detectors. Despite significant progress in the
understanding of QP dynamics, pinpointing their origin and decreasing their
density remain outstanding tasks. The cyclic process of recombination and
generation of QPs implies the exchange of phonons between the superconducting
thin film and the underlying substrate. Reducing the number of substrate
phonons with frequencies exceeding the spectral gap of the superconductor
should result in a reduction of QPs. Indeed, we demonstrate that surrounding
high impedance resonators made of granular aluminum (grAl) with lower gapped
thin film aluminum islands increases the internal quality factors of the
resonators in the single photon regime, suppresses the noise, and reduces the
rate of observed QP bursts. The aluminum islands are positioned far enough from
the resonators to be electromagnetically decoupled, thus not changing the
resonator frequency, nor the loading. We therefore attribute the improvements
observed in grAl resonators to phonon trapping at frequencies close to the
spectral gap of aluminum, well below the grAl gap.
