Absorption is what you are doing. Sonics will degrade if you cover with silk. So, a better bet would be to cover with speaker grille cloth. Check Parts Express for their black, 70 inch wide stuff. Inexpensive and very effective. Also, have you considered moulded form rollers that you could also cover the wall. The roundness creates better absorption. Here is a link.
http://www.performbetter.com/detail.aspx_Q_ID_E_4918_A_CategoryID_E_235
Tweak, tweak and more tweak.
http://www.performbetter.com/detail.aspx_Q_ID_E_4918_A_CategoryID_E_235
Tweak, tweak and more tweak.