I figure the Audio section would be the best place to ask about sound insulation. I'm gathering supplies for a major soundproofing project - planning to lay down 1-1.5lb MLV on a 1/8-1/4" foam decoupling layer, below the existing factory insulation.

1. I get the "limp, heavy, airtight" general guideline, but not sure how to apply this to deciding what foam to use for a decoupler. Stiffness matters - less is better - but what about density? Should I use closed-cell for durability or other reasons, or open-cell for absorption?

2. What's the best way to layer MLV and foam, in areas that allow a thicker stack of materials? Say, for two layers, should I do foam-MLV-foam-MLV, or foam-foam-MLV-MLV?