There is info on insulation for headliners around too. I didn’t pull mine out so I haven’t got to that yet. It is different stuff I think, lighter and uses other materials. Detroit Muscle sells some dash insulation kits.
I did a lot of research on sound proofing, but found it hard to implement in a car. As I understand the idea is to use different densities of materials sandwiched. When the sound wave is traveling through a low density (like foam or Fiberglas insulation) and it comes to a higher density material (like fiberboard), part of the sound is reflected back at the interface of the materials. I believe the magnitude of the reflection is based on the ratio of the densities. If you were to have multiple layers, the amount of sound would get smaller and smaller as it went through the layers. Some of it would be absorbed and some would be reflected. Some of the materials are pretty pricey and it looked a little dicey, so I didn’t pursue it.
Well, I have probably told you more than I really know. There is a lot of good info out there. I included a few pics of my dynamat install.
View attachment 1715275520
View attachment 1715275521
View attachment 1715275522
