From Floyd Toole's Sound Reproduction:
As shown in Figure 21.4, drapes can also be effective sound absorbers if they are of the right kind, and that is simply described as “heavy.” Heavy velour, the heavier the better, is probably the most popular choice because in its draped form it constitutes a resistive absorber of signifi cant thickness. Other heavy fabrics with the right porosity/fl ow resistance also work well, but lightweight, open-weave fabrics (easy to blow air through) are useful only for decorative purposes.
FIGURE 21.4 A fuzzy curve showing approximate absorption properties of heavy velour drapes, draped to one-half of their fl at area. The drapes should be hung on a track located 4–6 in. (10–15 cm) from the wall to ensure good low-frequency absorption.