[MatrixDweller]

You'll have to take extra care to soundproof the door jamb as well. Sound can flank around your massive door through the cracks in the shimmed door jamb. You'll want to seal the crack with green glue or elastomeric caulk. You'll also need a high end gasket and sweep system.

SOund is going to penetrate into the other room through the weakest point in your wall/door system. So if your door was super high 75 STC, your wall was 65, the ceiling that is shared between both rooms is 65, but the jamb around the door is only 50 then you will hear sound coming from the door jamb, albeit ~50dB quieter than in the noisy room. That also means that using a door that much over the STC rating of everything else is pointless.

If you do go for a heavy door then you'll also need to make sure the jamb is extra solid so that the extra heavy door won't pull it out of place. Standard finishing nails are not going to do the trick.

 

[MatrixDweller]

Yes the door frame is a good 10 inches thick, it is an OLD home. Some things are built like and others are WAAAY overkill.

Odds are that the molding is 10" wide which covers the jamb. It's most likely good and solid if it's in an old house and possibly made from hardwood. If you're lucky there is a solid timber running up from floor to ceiling that is thick. It's still a weak point where it joins the wall for sound flanking through and my point on the weakest link still applies.

If it's a plaster wall...is it filled with insulation of some kind, or is it hollow? Same with your ceiling and floor that are shared with the other room?

Also if the jamb has never had a door on it you'll have to make sure it's square and strong when you put the door on it. If you plan on making a heavy soundproof door then you'll probably need to use 3 or 4 heavy hinges with long screws that dig in to the wood framing.

A good door gasket is also required to maintain the the STC rating. Even a standard solid core door (like a Masonite Safe n Sound door) with a good gasket system in place will stop a lot of sound.

You'll want something to prevent the heavy door from being slammed shut also. If it's slammed it will put a lot of force on the wall system and could end up cracking your plaster.