Sound Absorbing Drapes

The result off a methodology using materials that are wanting in how sound energy is managed will certainly not be appropriate to create a accurate experience of sound energy being managed by materials correct for the role.

Glass Wool such as Rockwool at a Slab density @35Kg/m3 where the Batt is slipped into a cotton covering wi introduce fit for purpose materials to trial with. 

Do create the space between inner face of the Batt and the Wall. The bounce back is and important sound energy management. 

Reserve a bedding material to keep a Winter Chill more comforting to bear in the Winter

@hikerneil 

You should check out cellular blinds.

ghdprentice +1

ghdprentice is correct in that heavy absorption behind a planar speaker will overly deaden the rear wave and seriously dilute what’s great about a planar design. 
Many years ago in my first apartment I bought my first real pair of speakers, Martin-Logan Sequel II’s, a hybrid design with a 48” x 12” electrostatic panel and a 10” woofer in a sealed box. My apartment was of generous size with a soaring vaulted ceiling. But I could only pull the Logans maybe 2-3’ from the wall behind them which noticeably blurred the soundstage and caused a bit of midrange glare.  After some experimentation, I found some handmade tapestries in a shop on an American Indian reservation in northern Michigan. Though they were sized like a throw rug, about 5’ x 4’, they were meant as wall hangings. These were woven from knotted soft cotton roping into a strikingly original brightly colored design. The knotty texture provided some diffusion and the soft cotton offered a bit of absorption. Hanging one behind each ‘stat panel involved a simple pair of finishing nails into the wall. These original pieces of artwork brightened up the room and brought the Sequels into focus. 

Any drape absorbs sound.The thicker it is the more it absorbs.Stop obsessing.

I think it is fair to say that a room will be at its best when a microphone is used along with computer software to read room modes and learn what can be done within the space using absorption/diffusion..

The aesthetic of the selected treatment can't be disputed for the impression made on the selector's eye and the integration with the space's decor. Beauty is in the eye of the beholder.

Without Data being recorded, any explanation for the effect of a material used within the space for mode control will be conjecture. Even if the user of the room detects a change to the end sound presentation, there is also the risk that remaining within the room are modes/nulls and a speaker positioning that are not ideal.

It makes sense to get the space mapped to learn a methodology to treat the room..