Preventing noise between 1st floor unit and 2nd floor unit

[Shawn Mcenteer]

I live in duplex MFR, units are on top of each other. Floors are very old, 3" wide fir installed directly to floor joists. I specializes in wood flooring so installing, sanding etc would be done by myself. I want to sand and refinish existing floor but the concern I have is noise traveling between units. The property will appeal to luxury "A" type family. Currently the main problem will be noise on the first floor coming from the second level. What solutions have members used in the past to lower noise between units? I have high ceilings so installing wood on top of existing floor is possible but expensive considering I can refinish existing floor myself. I live in first floor unit so I can refinish upstairs floors then attack noise issue from my unit via sheetrock layers or sound dampening fixtures that attach to joist (large project and not cheap).