I need some advice on some minor(?) issues regarding an independent wall I'm building for sound insulation. It's going to be a DD on a 2x4 frame with mineral wool insulation inside.

The issues I'm debating:
- which should I use for best sound insulation: glass wool or rock wool?
- does the density of the mineral wool affect performance? (it surely affects it's price!)
- I'm going to use a 5/8 +1/2 drywall. Does the order matter? Which should be on the outside?

Use Mineral Wool as the best option. Order does not matter in terms of hanging from a sound perspective but personally I would hang the 5/8' first.

The use of fiberglass insulation or mineral wool will do a reasonably good job in absorbing the higher frequencies, but will do nothing for the lower end below 125hz. In addition, building one wall may not be enough as flanking noise may be an issue. If you're going to punch hole in the wall for electrical outlets and electrical sockets, may I suggest that you offset these at a minimum (different wall cavity) to reduce a direct path for sound travel and seal the junction boxes with acoustical caulk.

Thanks!

Btw, it's gonna be a room-in room, so I'm not worried about flanking noise.