Where better soundproofing is required to made to an existing wall or designing new walls in which a door is a part of that wall, consideration must be given to improving the acoustic requirements of the door/s as well as the walls.
Typically solid core doors even those of 40mm thickness only achieve an acoustic rating of 31dB with acoustic seals.
A standard wall with 10mm plasterboard either side of 90mm wide studs achieves an acoustic rating of 34dB. This is greater than a 40mm solid core door.
Therefore where improved soundproofing is required all the building elements ie, walls, ceiling, floor, windows and doors must be considered and have similar performance as noise will take the path of least resistance.