want to build an apartment downstairs without noise coming thru floor
If you could insulate the floorboards from the joists, that would cut a lot of noise. But since the floor is probably already in place, the best you can do is to put padding and carpet above, and insulation (fiberglass batt or other) above the downstairs apt. wallboard ceiling. You can also attach acoustic ceiling tile to the apt. ceiling. If any walls also are shared between upstairs and downstairs, you'll want to insulate those, too.
You can best do this by isolating the floor from the ceiling by adding a new set of ceiling joists and have the apartment ceiling structurally separate from the floor. The ceiling joists would go up between the floor joists with the bottoms about an inch below the existing floor joists. Dampen the space between the joists with fiberglass batt insulation and also use a good dense sound board on the bottom of the new ceiling joists. The gypsum board ceiling can be screwed through the sound board into the new joists. This may not be the cheapest way but it is pretty effective way of reducing sound transmission through a floor/ceiling.
