Absorber behind corner bookcase

The bag may not be helpful. You'd be better off with a pile of clothes. Almost anything works to some extent. One of the best room treatments ever is bookshelves loaded with books. A bag of insulation sounds ok if you're in college. If not, try some Auralex bass traps. I can notice where I need treatments such as bass traps. Is there some reason for you to believe this is a trouble spot?

more important then buying new gear and most ignored by all is room treatment. Your idea of just stuffing is in the right direction but off a little. Place it tri corner v shape with an air gap"cavity". Also books are great for both damping and diffusing along with lps.

Basically, it is a flat bookshelf filled with books against a corner that leaves an empty triangular empty space behind. I have been storing some skis behind there but I sold them and I thought of adding some kind of absorbing insulation behind the bookshelf. No one would ever see it back there as it is obviously hidden by the bookshelf itself.

From what I hear, leaving the air gap behind the bookshelf is the best method, correct?