How to connect silencer in attic to room below?

The attic in my double-framed garage will be the host of my silencer boxes (one in and one out) which supply and/or remove air from the completely new framed Inner Room below. I'm realizing, however, that the ducting will need to connect to the inner room below somehow. 1. How does one connect the hole from the Outer Room to the Inner Room's ceiling? (Note: The attic floor is the ceiling of the outer room.) I was planning on using a short run of flexduct (maybe a foot or so?) between the two structures but it seems to me that it needs to be in some 'beefed-up' box so that the void between the walls isn't blasted by sound. Seems to me like this beefed-up box would need to be either: a) supported by the structure below and connected "fire block style" to the structure above it or b) connected to the structure above and connected "fire block style" to the structure below it When I say "fire block style" I mean an actual physical connection made of insulation and/or fire-rated caulk only. (I.e. No wood connection.) Thanks for any advice.

Corret, the connection between the inner and outer shell needs to remain decoupled and you need enough mass around the penetration and associated duct to match you isolation mass. So if you used 3 layers of drywall in you room, go ahead with 3 layers on the extension to be on the safe side.