Soundproofing floor? (1 Viewer)

[GaryM]

I need to soundproof the floor in my Home Theater room, which I will be converting from the present front room space. The floor construction is 4X6 timbers on 4' centers, spanned with 1 and 1/8 inch tongue-and-groove plywood subfloor, with wall-to-wall carpeting on top. Underneath this floor is about 12" of crawlspace, with three perimeter foundation vents which presently transmit a LOT of bass from the subwoofer, making just the low frequencies all too audible for my neighbors.
I would like to remove the carpeting and add soundproofing on top of the subflooring plywood. I believe the problem is the large span between the floor joist timbers. I grant you 4' is a lot, but many houses are built in this fashion in California because of earthquakes, with heavy joists tied into even larger perimeter timbers with large steel brackets, which in turn are bolted to a cast concrete foundation.
If posssible I would like to stiffen and soundproof the floor from above - I don't have any room to work underneath because of the shallow crawlspace. Any ideas?
Gary

 

[Orin Dym]

BaSo4 (Barium Sulfate) loaded vinyl, 3/8 inch thick. The stuff weighs about 1 pound per square foot, is easy to install, will not raise your floor height much, and is effective, but expensive. I used it on the only wall I share between theater room and house interior. Measurements before and after indicate about a 20db drop in transmitted sound.