I am getting ready to build a 2 story 2,400 sq ft. duplex with a 1,200 sq. ft. 3 bed 2 bath downstairs and an exact copy stacked on top, the owners want extensive sound proofing as they are both musicians who make their living playing the Viola with hours of daily practice.
I have been researching how to get the most bang for their buck (or least bang transmitting!)
4 things that one can do to mitigate sound transmission,
1. Decouple, this one yields a lot but not sure how you can incorporate this in a vehicle.
2. Mass, this is one of the most important things to do as the more mass an item has the less it can pass sound (except the lower frequencies)
3. Damping, works very well in reducing transmission of sound through the absorption of vibration energy that would turn a surface into a transducer
4. Absorption, works best with mid to higher frequencies and is a very important part of an entire system to mitigate sound transmission.
The sound deadening mats typically used in automotive work well because they address 2 of the 4 items very well, they add mass with their composite of a thin metal sheet and the layer of vinyl, this property also adds damping.
There are few cavities you can fill with absorptive materials (doors being the only ones that come to mind without losing space) and i'm not sure you can get enough in to make a difference but I don't know?