Double Leaf ceiling?

Just a quick question, would one normally consider the sheathing of a roof and shingles and whatnot as an outer-leaf and the ceilling hanging under the roof trusses as the inner-leaf of a two leaf system? or is the "attic" space generally large enough to be considered like a separate room itself? I am just wondering if I have a existing ceiling and under it build a "room within a room" with it's own suspended ceiling, would the actual roof sheathing not constitute a third leaf? Regards, -jER

shingles or sheets of iron wouldn't be considered a leaf as they aren't airtight so don't cause a spring in a MSM setup.

I am just wondering if I have a existing ceiling and under it build a "room within a room" with it's own suspended ceiling, would the actual roof sheathing not constitute a third leaf?

Even if the roof deck does act like a third leaf, the air gap in the attic space is so large that the resonant frequency of the system would most likely still be plenty low enough that it wouldn't be an issue. - Stuart -

Ok, that makes some sense... I have a similar question regarding a closet in my live room and my HVAC room, but it will wait until I get a sketchup of the interior completed. Thanks Stuart!