My 12.5' x 15' listening room has an 8'-wide patio door on the long side. I was concerned that the large glass area might be hurting sound quality, and even got an estimate on a sound-absorbing curtain—about $600. Then I watch this video1, and now I don't worry about it any more.
Lots of curtains absorb sound. You can actually hear it in the store. Put your ear right up to the curtains/carpet or wall covering. Lots cheaper than $600. :)
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