Flanking through roxul?

I have been steadily working on a room within a room in my garage.... origin thread http://www.johnlsayers.com/phpBB2/viewtopic.php?f=2&t=11806 Wood construction, 2x4 16oc studs, communicating doors I've finished my fire-blocking and passed inspection :D , but I thought to do some double checking before I start sealing it all up. Where I previously had no vibration transmission between frames, I am now getting a very muted, barely noticeable transmission. :( Fire-blocking is Roxul AFB 40kg 2 inch, cut into strips just slightly bigger than the 1 inch gap between frames at the corners and top plate of inner room. I've isolated the section that seems to be the culprit, removed the roxul, and the transmissions stopped. I cut it down further and reinserted it into the gaps, however I'm still getting some transmission. Source of the vibration is a standard utility vent fan attached to the outside framing to suck air through ducting (though I also performed a test by tapping on the inner frame with a hammer and feeling the outer frame). Like I said, this is very faint, but its still noticeable. I feel like I'm about to lose my mind over this. If I cut it down anymore I can't see how it can stay in the gaps on its own or create an effective fire stop. Have I possibly just compressed it too much in another section (though when I removed this area it seemed to fix the issue)? Is there to be some expected mechanical vibration transmission through roxul under slight compression that is just in the realm of acceptability? will sonic vibrations produced inside the room have the same effect? HELP!!

first step - decouple the fan so it is not passing any vibration. then ensure the ducts etc are also decoupled from the interior room.

Thanks for the reply Glenn The fan assembly (thru wall vent fan that is mounted in its own box) is right now attached to the outside frame, with flex duct snaking to my silencer boxes attached between the joists of the inner frame. Finished product will be attached with some gaskets to minimize some of the vibrations in the outside frame. While I'm using the vibration as an easy constant to help identify that I've got some flanking happening, I first noticed and confirmed by tapping on the outer frame and feeling the inner frame with my other hand. The vibrations from both the fan and tapping didn't occur until my fire-blocking was installed. So I guess my main question is....will there be a certain amount of mechanical vibration transfer through roxul under slight compression? or do I have to rethink my fire-blocking? Or very likely, have I just not done it properly and need to keep redoing it until I get it right?

could you post a photo of the fire blocking you've built?

I'm having a problem getting my pictures uploaded.... Basically I have a one inch gap all the way around. I cut the 2 inch roxul into strips that are slightly wider (maybe a quarter inch?) and stuffed them into the gaps so that the 2 inch depth of the roxul is perpendicular to the 1 inch gap, if that makes sense. I used a flat spreader (like for icing cakes) inserted between the roxul and the frame to make sure the roxul was puffed out to its full dimensions and not hung up on a splinter or rough patch inside the gap. I've done some tinkering and it looks like I've got enough surface area along my gaps to redo my fire-blocking like so... from the top side of my walls, apply just enough pressure so a small percentage is actually inserted into the gap with the rest of it sort of "resting" on top of the framing spreading over the gap. Applying a bead of adhesive along one of the surfaces to help hold it in place. I definitely have the surface area in the vertical corners to adhere the roxul to one frame with the other side of the roxul pressed up against the opposite frame. I realize that this won't keep it in position long term, but once the cavities are slightly over filled with the pink fluffy stuff, it should help keep them in place right? I don't know if these methods would meet "code", but I've already had my inspection and the inspector barely looked at the fire-blocking and certainly didn't take any notes. Not suggesting I remove it entirely, but maybe I don't need to be quite as precise while redoing it? Thoughts? I'll try and get my pictures up again and maybe an example of what I mean for my alternative.

Problem solved.... I just kept massaging it until I seemed to get just enough roxul inserted to seal and used some "bridging" of the gaps attaching roxul with some adhesive where I was sure it would stay in place. I'm still curious about the transmission of vibration through slightly compressed roxul. I was under the impression from research here that 20% or less compression was ok, but I got noticeable transmission of tapping vibration from one frame to the next with less than 20%. I did notice it seemed to depend on the orientation of the roxul. Since the roxul seems to be layered, if the compression went against the grain, the vibration was worse. If I oriented it so the layers compressed further together, it seemed to lessen the transmission.

mineral wool tends to be trickier than fiberglass in this regard due to length and related flexibility of the fibers.