I've never needed to use ... but if I did, I would try the "Block out" curtains. They are for blocking out the light... and so they have a rubber backing. I think that would do wonders over just fabric alone.
Good Materials for Curtains
Has anyone made heavy curtains specifically to add absorbance to a room, and (of course) to block the reflections of glass? If so, what materials did you use. This is such a simple and common acoustical technique, yet I can find little written about what types of fabric (etc) are best. Thanks for your experienced suggestions.