Sound Absorbing Drapes

@hikerneil  Can you tell us if your Martin Logan speakers are electrostatic or typical box-type that the company got into more recently?  I ask because I personally wouldn't treat the front wall behind the speakers the same way with such different types of speakers.  With my SoundLab electrostatics, they sounded best (to me) when pulled about 5feet from the front wall and it was left untreated with just a wooden front wall to reflect the rear firing sounds.  But with my front firing box speakers (Acoustic Zen) the front wall is well treated.

Have you measured your T30 decay time as a large thick curtain will reduce your mids/highs decay times and could lead to a "dead' sounding room that is not musically engaging; it could dilute the benefits of the other treatments you have done elsewhere.

I've used sound absorbing drapes for a decade to attenuate the music coming from  my audio room.

They do well with high frequencies. But anything below about 300Hz or so is only slightly attenuated, if at all. 

When listen I to music coming from my audio room from  outside the room, I hear what sounds like a subwoofer. This effect gets more and more bassier as I move away from the drapes.

Fiberglas traps made properly, such as the ones from Real Traps will do a far better job with frequencies down to 100Hz, below that, corner traps definitely help but still don't fully tame room resonances. 

M conclusion is that sound absorbing drapes make great room dividers.

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.