I 2nd the Secondskin Audio damping products. I've used a lot of what they sell, from the old Damplifier (equivalent to what you'd get at the showdown website) to Damplifier Pro to Alpha Damp to Spectrum/Spectrum Sludge. I also used a lot of 3M's 4274 NVH damping compound caulk.
It's all good stuff, but despite rosy marketing, each behaves differently and each works at its best under certain circumstances which they don't spell out. It *is* also possible to make things worse, typically by covering too much area, although if you don't go overboard you'll have a significant improvement with any of their products, even without performing any acoustic tests.
A piece of sound advice for you - if you use the "sexy" looking black Damplifier Pro, cover the black surface with Aluminum foil when you're done, or the black surface will act like a window and radiate heat into your vehicle in summer and allow the heat out in winter! I measured this and it's for real! They won't tell you *that* on their website!!
I'd also recommend going over the painted surface of your panels with some Acetone or something similar, before applying the damping, using cloth which doesn't leave fibers. You *don't* want any surface contaminants, or the damping material may fall off! I went so far as to spray fresh paint so I had no chance of contaminant issues.
The thing to keep in mind at all times is that as you add damping the resonant frequency will drop, so even as you reduce the amplitude of a nasty peak, you may move it down on top of another lower frequency peak! Don't just slap this stuff on at random; keep listening to the results as you go. As you complete adjacent panels, go back and test what you already did; it *will* have changed!
I took thousands of precision acoustical measurements in my van as I modified it, and tailored the application of each material to do the most good. I'm still working on the front end and driver area.
Please note that I applied more damping than absolutely necessary in many areas; the modal test technique I used, while well established for aircraft, was experimental on a van, and it's far harder to remove this stuff than to keep going! ;0)
In most cases, I'd recommend someone without test equipment or much patience just apply diagonal strips of damping material across the most obnoxious areas of each panel; all you really need to do is break up the worst resonances by redistributing the vibrational energy across the panel.
Honestly, based on my results, the most bang for your buck would be to add additional bow and strainer braces to break up the resonance of the largest unsupported panels. Then far less damping need be applied. Position the additional braces with uneven spacing, to reduce the odds of having multiple panels resonating at the same frequency, creating a peak.
You're in luck; just tonight I uploaded about 300 pictures to my Photobucket account! Here's a few pictures of some of what I've done inside my van, in case it might give you some useful ideas:
I custom installed damping materials, up to 3 layers thick, in every single major and minor panel:
I added a bunch of extra bows; the only practical way to deal with the genuinely obnoxious resonances of the roof. The very worst resonances were perversely enough directly over the driver and passenger's heads!!
Then I installed custom trimmed pieces of thin foam insulation between each bow. This was a lot of work; the Dodge bows aren't a uniform thickness end to end, so the number of layers varies. There were quite a few layers, to get up to flush with the bow surfaces...
A final uniform layer of foam:
An example of some of my custom damping on a wall panel; there are three types of material in use here:
I added numerous additional strainers and covered all the damping with a layer of Aluminum foil to block heat radiation:
This is poly-bagged fiberglass insulation. I sealed the bags with an impulse heat sealer to keep the fibers permanently confined:
This is how the damping on the floor looks. It's unnecessary to apply damping materials over the cross braces. The area over the exhaust and end of the transmission were perversely transparent to noise... it required a lot of attention to deal with them, including treating the noisy heat shields!
I put a layer of Reflectix over the floor damping
Then a sandwich of plywood and EDPM rubber, then more plywood went in. I learned this noise reduction technique from our own Vanner Pex, who works at a commercial bus factory in Europe. They use mass loaded Vinyl, on the order of 1/4" thick, which would be prohibitively heavy for a van. EDPM rubber works nearly as well.
The EDPM rubber actually extends up the walls 6-8". I could make a swimming pool! No need to worry about spills getting under the floor.
The area where the seats bolt down is the only area where the floor is pinned down to the van's floor pan; it floats everywhere else; a really cool trick I learned from our own Vanner Doc 2%. This helps absorb and dissipate quite a bit of vibrational energy.
It's *much* quieter inside now; and is quite temperature stable. I do love my A/C!! :0)
Here's a little info on the sort of acoustic testing I do. It's got it's heritage in aircraft noise reduction. I have copies of many of the original research papers.
A good test hammer for this purpose could make use of a piece of Sorbothane like from a gel shoe insert, taped over the end of a small hammer. Mine uses about a 3/8" thick gel I obtained from McMaster Carr. You wouldn't want anything much firmer or you could damage your paint or warp a panel!
Here is a 24 band per octave test plot of one single point on a large panel on my van before damping:
And after the application of my test-driven damping:
I did pretty well didn't I? ;)
Another before and after: