Amazingly, that thing does give some noise, but it's the one in the back of the room that I'm more concerned about, given it's proximity to the back row.
The back vent is a 6" acoustical flex duct, with lots of bends, going into a ~22"x18" DD+GG box with it's interior lined with linacoustic. I've taped the lip of the 6" hole's opening into the box to provide as smooth a surface as possible. The bottom of this box opens to the room's ceiling near the back wall, almost immediately behind where the the PJ would be mounted .
Looking straight on:
I've already tried a few things to address the problem, including putting two 340 CFM fans (one on the supply, one on the exhaust) on a speed-controller (this helped) and trying to isolate the back fan from the joists (this didn't help). The one thing I haven't tried yet is to stuff lots of pink insulation around the fan, but it seems obvious that problem is too much CFM. The ultimate fix will be to open the ceiling and split the line but, before doing that, I'm wondering if anyone knows if carpet/linacoustic/chairs would help...do you know?
This is coming up now because I finally got the door with bottom-closer installed caulked everywhere. At this point, the room is sealed pretty tight, so I can no longer hope the sound coming from outside the theater.
BTW, the carpet is scheduled to be installed next Thursday. I can delay it, or let the installation proceed and find out the answer to this thread's question directly, but then, if it turns out no good, I would have to be very careful covering the carpet before tearing open tearing open the ceiling
Thanks for your post, hopefully the forum will point me in the right direction...