AVS Club Gold
Join Date: Nov 2002
Location: Upstate, South Carolina
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I don't mean to oversimplify or to speak for Dennis as he and the other pros can explain this far better than I can.
What is going on is, as you said, your subs are trying to fill the entire basement. This is because the low tones produced by your subs are passing straight through the walls. The insulation you installed did very little to stop this but as you now know, it is dampening the resonance within the wall. Although there are great benefits outside the room by isolation, what is often understated are the benefits inside the room with isolation. The most accepted and easiest way to do this "sound containment" is double layers of sheet rock inside the room. You will also hear improvement when you install the sheet rock on the outside of the wall.
Since you had only one layer of sheetrock over the studs inside the room and the other side was open, you essentially had a giant drum the subs were striking with each note. This was producing the resonance you had and still have to some extent. Until the wall is "rigid", this resonance will still occur to some degree.
For now, I would concentrate on installing the sheet rock in the hallway to close the wall. I could be wrong and the pros can lead you better than I but at the moment, it's probably where you will get the best "bang" for the buck.