Rigid foam for window plug
I am currently setting up my home theater room. It's in a half sunk basement, which procures some decent insulation (to the outside) but has a large window (72*35). The window has dual pane glass but it is still the weak point. I'm building a frame for it and will install some removable window plugs. They'll be about 3 inches thick, including the 2 wood faces.
I'm wondering what would be the best insulation material to use between those faces in order to get the best soundproofing for the window.
I've used rigid foam insulation from R-Tech on my garage door (that was for heat protection), it's available at the local HD, in 1.5 inch thick boards. I also have a roll of R13 pink insulation, which could fit, compressed to 1.5 inches or 2 inch, depending on the thickness of the wood.
So, rigid foam vs pink R13, would it make any difference?