We ended up adding a prehung 30" hollow-core interior door between the two bedrooms. However, the door provides NO soundproof at all, it's very easy for the kids to stay awake when a conversation is being held in the adjacent room.
We want it to be sound proof, so the thought of getting a fiberglass exterior door came to mind. Is that overkill? Are they any more soundproof than a solid core interior door? Also, there won't be much traffic going between the two rooms, so the extra weight of the exterior door would be ok.. we just want it to be very sound proof. I'm also thinking of adding some insulation behind the drywall above the door..
When sound-proofing cats chime in on these questions, they always say "Mass, mass, mass." So, yes, a more dense door will stop more noise. What, though, about the top, bottom and edges of the door? What are you going to do to stop noise blasting right through the cracks? That needs addressing. Tacky and gross, but can you hang a sheet of sheet rock on the door you have, and paint it? EPDM gaskets may work for the top and bottom cracks. And, while you are at it, what about the walls around the door? Insulated? Double sheet rocked? Etc. Good luck. j