Oh yeah, I've done so much reading on the actual construction of baffle boxes, but was unclear on the port sizing, but that's cleared up. I'm certainly interested in any feedback you or Greg have.
Technically speaking, the more abrupt cross sectional area changes there are, the more insertion loss you will achieve. However, with every abrupt change in CSA or direction, comes an increase in static pressure. If you increase the static pressure in a system, you will need to upgrade your inline fan to one that can handle the extra load. With that comes higher electricity consumption. So, there is always a trade off just like every other facet of studio design!
So, to more directly answer your question, the CSA between your inner and outer leaf silencers can be the same as the duct feeding the system (the small CSA). You will yield great results if your impedance mismatch is greater than a factor of 2. Therefore, if you do have the space to make the silencer paths 4 times or 16 (this is extreme of course) times larger than the feeding duct, do it. Also, in my experience/real life tests, 1 great big box is better than several small ones even though they have the same equivalent cross sectional area. So if you have to use a few smaller boxes, add 1 or 2 extra baffles and make their path as long as possible (picture really long skinny silencer boxes).
Another cool trick is to Y split your silencers. You'd be building a normal silencer that has twice the CSA in the box path, but you'd add an identical one on the other side. This will ultimately quadruple your CSA and halve your air velocity. Killer, yes. But realize that it will be eating up twice the space and twice the material.
Would Sound Seal duct liner work? It's pretty hard to find JM LINACOUSTIC R-300 that's actually for sale to non HVAC contractors.
That stuff looks fine, yep!
One more question about construction. I'm going to add 2 layers of drywall (+ backing rod/caulk) in between joists against the upstairs sub-floor. Should I block off the spaces above the wall top plate with a similar treatment, or would this cause problems (like a three leaf type of situation)?
As Paul mentioned, it all comes down to if you are going to actually isolate your space. If you want isolation and are able to block off only the cavity of your fireplace, you're never going to get ANY decoupled inner leaf, period. You'll always have flanking through the fireplace. Therefore, the best isolation you can achieve will be that regulated by the Mass Law equation. If this is the case, then your inner leaf sheathing (the two layers of drywall) will be the only thing stopping sound transmission (note, you will always have structure borne sound transmission). That means you don't have to worry about beefing up the bottom side of the subfloor above.
Greg
