Soft ceiling or cloud?

I've a ceiling with the rafters still exposed. I'm going to insulate it and finish. The room is 10ftx20ft with the ceiling sloping up to an apex of 10ft tall at the midpoint of the 20ft wall. I was thinkin about just makin the entire ceiling a "Cloud" with 703 spanning the entire ceiling 'tween the rafters covered by burlap or sumpin soft. Question: Is the airspace between a conventional cloud and the ceiling somehow involved in the acoustic benefit, or is the cloud's bottom surface the primary "purpose" of the cloud (and so my "ceiling plan" would properly function)?

yes, the space "behind" broadband absorbers can make a difference in how much low-end it will absorb. I'd be carefull adding too much broadband between your rafters or you'll kill the natural reverb too much. aka, make it too "dead".

I definetly don't wanna "kill" the room, and I've been reading about diffusion to help defeat unhealthy reflections while preserving some of the room's brightness. Could I alternate reflective & absorbative covering surfaces to help grab the lows but alterataively absorb & reflect some of the highs? My motivation is to keep costs down, but make as effective of combinations/compromises as possible.

yes, you sure can.

Our hears are most sensible to reflections comming from side walls. So i'd say Kill it make it dead this is a great space to use for that without too much side effects, fill the space between rafters with insulations then make a huge cloud ceiling at the desired height also filled with insulation. A ceiling like that would provide control down to lower freqency without any lost of floor space. Then if you think it's too dead mess with your walls