The only things that are going to stop sound are mass and air space. In various combinations.
So, a drywall wall has both. It has mass in its two layers of gypsum board. But a single layer wouldn't do squat. Nor would two layers stuck together to form one thick one. It's the air between that kicks it up to an effective level. Of course you don't need the air if you have a LOT of mass. Like a concrete wall. You probably don't have that.
The sound at some freqs bounce right off that first layer of drywall. Good. Isolation. At
some frequencies. But other freqs go right through. Lower ones. The waveform is larger. It actually moves the drywall and some freqs get attenuated there. Some don't. So it now travels through a locked airspace. Then it has to hit another piece of drywall. This system described above needs to be near airtight to work.
Sure, some sound still gets through, like a kick drum maybe. But you're pretty good.
So why not a hollow door then? Isn't that like the wall you just described? No it isn't. The walls of the door "system" are too thin. So go all mass here. Solid door. Cheap and easy.
but remember what I said about it needing to be airtight for that system to work? Right. So... any little cracks between door and jam are just going to render that solid door ineffective. You've got to make the system airtight. Hence the outdoor threshold. Or shove some towels. That'll help. But a solid wood threshold, and cheap foam weather stripping? Airtight.
Mass and airspace. Airtight.
“The truth is the whole.”
- Georg Wilhelm Friedrich Hegel
"It is easier to discover a deficiency in individuals, in states, and in Providence (and in pop songs), than to see their real import and value."
- Georg Wilhelm Friedrich Hegel
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