"drum room that won't disturb my neighbors"
Ah. Way to jump in with both feet there for your first attempt.
In general, whatever the reasons why, Rod's book's advice is what works. Follow it blind regardless of understanding, and it should be ok.
Variations on Bert's work too.
Such as a Paul Woodlock.
My hypothesis (mostly guesses, no facts, no studies, never seen anyone smart write anything like this) about air ducts is this:
1) the size of the duct relative to the size of the walls is small in terms of area, so only a % of the sound energy gets to the duct in the first place. Admittedly this is only a drop of a few db.
2) think about your house. If someone is talking in the basement, and you're up on the 3rd floor, you hear the low frequency noise of hammering transmitted through the framing of the walls, you hear the furnace fan and the wind of the duct, but you don't hear the voices. Why not? Someone may know, but it doesn't matter.
3) high frequency sound is fairly directional, and if there are a few bends in the hvac duct, particularly if the duct is round rather than square, then the sound is reflected around and back onto itself dramatically lowering the dBs.
4) low frequency sound is transmitted at least as much through the framing as through the ducts.
"The only thing we regret in life is the love we failed to give."
"Be a rapturist -- the backward of a terrorist. Commit random acts of senseless kindness, whenever possible" - Jake Stonebender