Actually cotton/denim (cut up jeans material basically) insulation is pretty good, but usually too soft. Also not "preferred" as there are products made specifically for acoustical treatments...

I know a few things about movie poster acoustical panels. While I am pretty quiet around here, I am the resident "expert" over at AVS...

Check this thread out... Start around the middle as things have greatly improved over the years. You can get a great improvement in room acoustics for really not a terrible amount of money, and they look cool too (don't have to be movie posters either, can be anything)...

DIY Custom-Printed Movie Poster Acoustic Panels - cheap!
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2-M60s, VP180, 8-M3s, SVS 20-39PCi, DIY Sub, 8-Shakers, JVC RS45, Anthem MRX-1120