Air leaks are sound leaks. Apply compressible weatherstripping around jambs, add an adjustable threshold, and fill casing cracks with paintable acoustic caulk. Don’t forget the peephole and mail slot: foam gaskets, brush sweeps, and discreet draft blockers stop surprising whistling and chatter without altering building architecture.
A solid-core slab with tight tolerances raises STC dramatically compared with hollow honeycomb doors. If replacement isn’t allowed, add a dense layer like MLV or laminated panel, then perfect hinge alignment and latch compression so seals contact evenly, eliminating sneaky sound paths that defeat heavy materials.
A heavy doormat, cork or rubber underlayment inside runners, and a slim upholstered bench soak up energy from footsteps and dropped keys. Hooks and textile organizers add porosity, breaking up reflections. Together, these subtle textures reduce transients that otherwise barge beneath the door and into shared spaces.