Phononic crystals—elastic counterparts of photonic crystals—offer prospects for controlling the propagation of elastic energy, with potential applications in tunable filters, thermal management, and acousto-optic devices1,2. By modulating phonon transport, these structures enable dynamic control over thermal conductivity. Colloidal self-assembly provides a cost-effective method to fabricate phononic crystals with tailored properties 3,4. In this work, we present a time-resolved study of hypersonic phononic properties in two-dimensional colloidal crystals formed by self-assembled polystyrene (PS) nanospheres on a silicon substrate. We demonstrate that the hypersonic phononic bandgap can be tuned by modifying the interparticle interactions. Ultrafast pump-probe transient reflectivity techniques were used to investigate phononic modes across different frequency ranges 5. The experimental results reveal strong phonon insulating behavior, supported by finite element method simulations. We also identify an avoided crossing, indicative of strong coupling between the contact resonance of the nanospheres and surface acoustic waves. These findings highlight the potential of self-assembled colloidal crystals as tunable hypersonic phononic insulators, advancing their prospects for next-generation phononic and thermal control devices.