Background for Cabin Noise Reduction
 

a smattering of tidbits from Google Search:
noise transmission frequency closed cell foam automotive

http://209.85.173.132/search?q=cache:vSJr-BpmHT4J:www.caraudiodirect.co.uk/esupport3/index.php%3F_m%3Dknowledgebase%26_a%3Dviewarticle% 26kbarticleid%3D22%26nav%3D0,6+noise+transmission+ frequency+closed+cell+foam+automotive&cd=1&hl=en&c t=clnk&gl=us&client=firefox-a

http://selectproducts.com/ftpfiles/procon2.pdf

http://www.800nonoise.com/whispermat.htm

http://www.cascadeaudio.com/blog/treating_road_noise.htm
- I'd just use stuff from these guys if I were not enamored of an analytic perspective...

http://forums.performancecar.co.nz/viewtopic.php?f=7&t=40497

http://www.dow.com/PublishedLiterature/dh_0042/0901b80380042875.pdf?filepath=automotive/pdfs/noreg/299-50620.pdf&fromPage=GetDoc
- NVH solns from Dow Chemical, specifically for cars - could be promising

http://www.talkaudio.co.uk/vbb/showthread.php?t=58457



http://www.thomasnet.com/products/acoustical-foam-30675706-1.html
- how deep do you want to go into this stuff??

The above are selected out from the first 3 pages of the 35k search hits. Selection based on a very quick perusal of the link - no warranty assumed or implied; your results may vary; professional scientist driving on closed course, but go ahead and try it at home

related thread:

http://forums.pelicanparts.com/porsche-911-technical-forum/462108-how-install-rs-carpet-make-look-good-post4539851.html#post4539851

That's all good information. The STC values, and especially the sound absorption coefficients tell you a lot about the appropriateness for automotive applications. I did a lot of similar research of materials when I designed and built a dedicated theater in my home. For a theater, the lowest frequencies are the most troublesome and difficult to tame, but for a car it's probably the higher frequency transmission that needs the most attention.