I think you're probably right.Foot-wells and firewall (if you can get to it) would be the biggest area's I would hit. Other then that you really only need one layer of MLV. IMHO the firewall, doors, and roof... are the areas you should focus on.
I intend to do some test runs with several mics located very close to those areas to more exactly pin down the noisiest areas, and the frequency/amplitude of that noise at each location.
Unfortunately it seems that the normal Windows OS sound driver doesn't support multiple simultaneous mic inputs. The alternative is to use a multi-microphone interface intended for studio recording (too expensive) or the 3rd-part "ASIO4ALL" driver which evidently can support multiple simultaneous mic inputs with the appropriate recording/mixing software (though I'm still not sure which software actually works).
I wondered the same thing (Use double alternating foam/MLV layers or one foam/MLV layer that's twice as thick). So I asked Doug at CAE.For best results, should i just do CCF, MLV, MLV? OR should I make a sandwich and do CCF, MLV, CCF, MLV?
While I was waiting for his reply, I found the answer in these two publications:
I ran across a couple of technical papers you might find interesting (if you haven't already read them).
1. Handbook of Noise Control Materials
2. Vehicle Noise Demands Tough Solutions
These excerpts are particularly pertinent::
1. (@ page 3): "For the design of composites, mass layers are typically at weights of 1 and 2 lb/ft2 and the thickness of decoupling layers varies from ¼" to 3". The effective frequency range of these materials varies with the square root of the weight of the mass layer times the thickness of the decoupler (See Fig. 2B).The 1 lb/ft2 material on ¼" decoupler is effective only above 500 Hz, and is generally not satisfactory for noise reduction in boats, except for special applications.The 2 lb/ft2 material on ¼" decoupler moves the effectiveness range down to 350 Hz (this is a construction generally recommended when there is minimum space available for the composite treatment). For a high level of effectiveness the decoupler layer should be 1" or greater and for the highest effectiveness this should be combined with 2 lb/ft2 mass layer, as demonstrated in Fig. 2B. Greater thicknesses of the decoupler increase the bass frequency effectiveness, while heavier mass layers increase effectiveness throughout the entire frequency range."
Here's a chart:
2. (@ page 5): "The double-wall barrier construction acts like a spring-mass system, where the performance of the spring is controlled by its stiffness and thickness. The lower the stiffness, the lower the frequency that sets the double-wall construction into resonance, the point at which it fails to impede sound waves. Similarly, the thicker the spring—in other words, the more deflection it exhibits—the lower the double-wall resonance frequency. In a double-wall barrier system, the decoupling foam acts as the spring. So, the thicker the decoupler and the lower the stiffness, the lower the double-wall resonance frequency."
Both these advise the same thing: for best performance in the hard-to-control lower frequency range, use the most massive (thickest) possible MLV along with the thickest and softest possible decoupler: open-cell foam, which also happens to be a better sound absorber than CCF. (Please refer to the difference between blocking the transmission of sound and absorbing sound.)
High Resilience foam, which has a very fast recovery and bounces back to its original shape faster after compression, is the best foam "spring." The optimal decoupler is a lot less firm (<1psi @ 25% compression) than the closed cell foams (>5psi @ 25% compression) commonly used in aftermarket automotive composite decoupler/barriers. Also keep in mind that MLV is a "limp mass barrier" and its performance is partly related to how limp it is. Being bonded to a layer of foam makes it less limp that MLV by itself. Which brings up the next point: anywhere is is possible to hang the MLV over an open space (such as over the open space of a door), use MLV without any foam behind it, except where it is mounted. That said, any added foam also has the secondary benefit of providing some sound absorption, but a thin layer of CCF doesn't absorb much.
Based on what I've seen on here so far, I expect that some will warn you against using open cell foam. While I understand their concern, you might want to consider that auto manufacturers do not totally avoid use of open cell foam (e.g.: seats). Also, you would have the opportunity to seal all the surfaces when you install it.
Doug at CAE and Don at SDS seemed to think my suggestion of using a much thicker decoupler than the normal CCF was ridculously impractical.
While that may be true under the carpet, the fact is there *should* be that much room behind the dash between the cabin and the firewall and/or front wheelwhells.
Additional references linked in this post.
Also, there is a tremendous amount of related technical info here.
One other thing I can would like to point out: Most "ordinary" passenger cars and trucks have a lot of cavities. The luxury counterparts of those cars often have similar cavities filled with something that is generically called "body foam" (e.g.: 3M Flexible Foam and 3M Rigid Pillar Foam). While these two products are intended for repair (and are ridiculously expensive), there are some more affordable alternatives, FlexFoam-iT! Castable Flexible Urethane Foam (but not the mono-component spray foam like "Great Stuff").
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