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?

I would put a twin of the bookcase you have now in the corner on the opposite walls. Tom

leave an air gap using the acoustical material on all corners. The book case to it best to have an air gap.