Sound Absorbing Drapes

I would experiment with blankets or sheets before committing to the drapes. While dynamic speaker nearly always benefit from heavy damening, planar speakers are different. I used planar for over thirty years and often only a small patch of dampening was required on an unadorned wall. so I would experiment with different thicknesses of sheets, blankets... etc. To determine the level that it best...

An option to give a larger range of Fabric Choices to improve the match for the decor, will be to produce a lined curtain, where sandwiched between the liner is a Sheep Wool matting at approx' 14Kg/m3.

If a 10mm thick matting is selected, that would equate to 100 pieces at 1/m2.

I'm presuming that an area of 20/m2 is the maximum area to be covered by the curtain, which is to add 2.8Kg of 10mm Thick Sheep Wool matting to achieve the coverage.

If the curtains are wanted to be pleated when drawn, there will be a need to use strips of the matting with a small spacing between them to enable the Pleats to form.

This method will produce an exceptional sound-absorbing screen, improved over a fabric only, but will have added cost. The type of cost if a brand's product was ordered in to cover a similar area, would be quite expensive on comparison, be limiting in aesthetic options and possibly use an identical absorbent substance or an equivalent.  

Properly treating a room is the best bang for the buck all most every time.  If your aim for less reverb the material is key.  Most synthetic fibers are at least moderately reflective so natural fibers like cotton, hemp etc. are preferred to reduce liveliness.  Good luck, let us know how it works out.  

IF windows are involved, heavy fabric vertical blinds can be effective, and still allow some daylight when partially open, then easily more light, easier than opening draperies, or fully drawn open, that's good in front of sliding doors.

draperies: fullness is the term that means the amount of fabric that makes the folds, more fullness means more fabric like prior OP mentioned.

I worked with professional video and sound companies for 35 years and they all used a heavy velvet.  We hung it mostly off of trusses, but if you can find all or most  of the studs on your wall, you can use threaded black pipe and flanges. I’d suggest keeping it a a few inches away from the wall.

All the best.