Originally Posted by jwl
Whether or not a material sags really depends on the material you put into the bag. 705 will sag less than 703, which in turn sags less than rockwool. Acoustic cotton sags quite a lot.
Rockwool comes in all sorts of densities from flimsy cotton wad type stuff to extremely dense vibration insulation used below tram tracks. It's less itchy than glass fiber.
I use a product called "acoustic plates" and they're plenty rigid enough. They also have two different facings, a paper like (with black, off white or white colour) on one side and fibreglass facing on the other side. Both facings are strong enough to retain the fibers without additional wrapping.
Have done some traps that are exactly as the OP describes it - simply duct taped the sides, leaving the front/back facings as is. Can't get any simpler than that! The sides can also be sealed by painting them with thick paint. It's an option that can be orded in manufacturing. The penultimate simple traps are to get some fabric and wrap it up, using tape to seal it. No sewing, no frame, just rockwool, fabric and tape. The ones I have with "frames" is equally simple - I just kept the cardboard frame the rockwool ships in.. To avoid excessive deadening of the room.
Here's some pics:
With cardboard and plastic and everything left in place as it's shipped from the store, fabric added for visual comfort(basstrap):
Without cardboard and plastic(broadband):
Where was that picture again...? Ahh! Here's the ceiling hangers:
Picture taken a long time ago, the traps are too few and not in their final position. There's aprox 100 kilos of rockwool up there now.