Oh! You are an electric guitar player. I was speaking as a recording engineer. There is a difference.
Have you tried a Power Soak?
To keep loud sounds in you need isolated walls and mass. Brick walls or concrete is good. In an apartment? Good luck. Foam won't do it. Sorry I am not more helpful.
this is true...I have a garage that has an aluminum door blocked up with like 2 or 3 mattress beds, and it sounds like there is nothing blocking the sound, you need heavy heavy drapes at least or brick or concrete something heavy and dense.
Every one told Henry Ford that the V8 engine could not be done… well guess what he did it!!! I have done some experiments with different items commonly available and I have had some pleasing results BUT this idea is still in the infant stages.. so there is still a long way to go.. and yes I am a guitarist