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Gordon, I think it is called a torsion box , not a box beam per se. But, yes, you offer a great response to this question.
And, for me it stimulates the following question:
Can the size of the joists be made smaller when both sides of the floor are sheathed so as to form a "torsion box"?
My goal is to achieve a house made of torsion box walls, floors, roof, etc, so as to connect with Simpson Strong-Tie type rigid connectors.
My purpose is strength.
Aside from that, the question of sound transmission when partying:
I know in the old days the 30lb felt was the best one got. Has it improved any?
Hope this helps.