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A hollow core door is hollow. The front and back facades are mounted to a paper honeycomb or a few scattered blocks in the door's interior. Because the facades are thin and the door is hollow, a hollow core door does little to impede sound.
A solid core door starts with a solid door base (wood, particle board, medium density fiberboard (MDF), etc.) and adds to that the front and back facades. Because the interior is solid, a solid core door is better at blocking sound.
Acoustic doors are usually a heavy particle board, or equivalent, with a wood veneer. The added weight helps the door's transmission loss. More importantly, though, these doors are mounted into a frame that includes one or all of the following:
- Gasketing at the sides and top
- Drop seal at the bottom
- Cam hinges
Gasketing is essentially weatherstripping. It is usually a neoprene "bubble" type that compresses when the door closes, creating an air tight seal.
A drop seal is a gadget at the bottom of the door that, when the door closes, drops a seal down to touch a door threshold. The seal is, again, usually a neoprene "bubble" type. The threshold may have its own strip of rubber as well.
A cam hinge is a door hinge that, as the door closes, drops the door downward to make a better seal at the bottom.
These features close air gaps around the door, blocking sound and improving the STC rating by 5 to 10 points. In contrast, a typical door has a large air gap under the door to allow clearance over carpet and to provide air flow for the HVAC system. Gaps around the door may occur when bumpers are added into the frame to soften door slams. The air gaps created by these features overwhelm any sound reduction capabilities of the door itself.
A solid core door with gasketing and a drop seal is around STC 40.
For a quiet room target of 30-35 dB of unwanted sound, or less, a solid core door with gasketing and a drop seal does a good job of blocking most nearby sound.
If manufacturer claims are to be believed, fancier acoustic doors can reach STC 45-50. This seems unlikely. For higher STC values, music studios instead use a pair of doors in an air lock configuration. The hallway between the doors is carpeted and its walls covered with sound absorbing material. But for a single door, an STC of 40 is probably the most that can be expected. | 
