Seal gaps around doors, windows, vents, and outlets with acoustic caulk or weatherstripping
Add a solid-core door and door sweep
Install thick curtains or soundproof drapes over windows
Use acoustic panels on walls and ceiling
Add mass to walls with extra drywall or mass-loaded vinyl
Decouple walls or ceilings with resilient channels or sound isolation clips
Fill wall cavities with mineral wool or fiberglass insulation
Use a thick rug or carpet with an underlay on the floor
Place furniture, bookshelves, or other dense items against shared walls
Use a white noise machine to mask remaining sound
Cover gaps under the door with a draft stopper
Upgrade to double-pane or laminated windows
Isolate noisy equipment with vibration pads
Reduce reflected sound with soft furnishings and fabric materials