I posted this over on ImprezaWRXSTI:
Some information on sound and vibration that some of you might be able to use.
I've noticed that a lot of folks who apply, or paid to have applied, a Dynamat type product are getting a bad deal. They either just don't know how noise attenuation works or they are maximizing profits.
Here's a semi-technical explanation, from a Master's level Mechanical Engineer, on the proper way to install the stuff.
There are two ways to attenuate noise in a car.
1. Sound wave AbsorbsionAbsorbsion
2. Resonance reduction
This is when you have a fiber batt that actually traps and prevents sound waves from propogating. The thicker the batt the lower the frequency that will be absorbed. Thin batts absorb high frequencies. (The thickness of the batt absorbes up to the freqency that is 1/2 the wavelength of the batt thickness) The denser the batt, (normaly ~5 times as dense as fiberglass insulation) the more of the frequencies that are to be affected are absorbed. So dense, thick batts will absorb the most sound.
Body cavities or deap space will collect and potentially amplify the noise, much like a wind instrument or guitar. This is where this stuff works best.
This is what Dynamat and its clones are trying to do. Every panel in the car will have a natural frequency that it will vibrate at then excited; Doors, firewalls, pillars, roofs, etc.
simply: Frequency = SQRT [ Stiffness / Mass ]
Dynamat essentially adds mass to the panels to alter the vibration charateristics of the panel. They also break up the standing vibration waves in the panels. (denser the better)
It doesn't really add stiffness, unless you make a structure and glue or bond it to the panel to increase the cross sectional area.
This is the main acoustic differnce between the tinny sounding doors of a stock subaru and the bank vault doors of a lexus.
What does this mean?
You don't need to plaster the entire
inside of the car with dynamat. Simply put 1-2" strips of the stuff over about 20-30% of the surface of the panels, in a chevron, or checkerboard pattern. This will get you 95% of the sound improvement of a full application without all that wasted mass or expense. (Dynamatt is EXPENSIVE!)
In the body cavities (firewall, behind the cosmetic peices in the trunk and package shelf, etc.) is where you want to put the batt. This will prevent a megaphone effect.
You can also get a drum effect, where a closed cavity (trunk) will make a panel (seat back) vibrate. Applying either one will work there, although Dynamatt is normally easier. The batt will also stuff between a hard structure and a cosmetic panel to keep the panel from humming (like the rear parcel shelf)
All strips should be about 1-2" wide, AT MOST
So, for the most cost effective solution I would:
1. Chevron the interior wall of the door skin with about a 20-30% surface area application.2. Put a few strip on the interior door structure around the speakers.
3. A few 1-1.5" strips of the stuff on the rear shock mounts from floor to top in sort of a spiral pattern.
4. A few strips on the underside (or topside if you remove the cosmetic shelf) of the package shelf.
5. A few diagonal strips on the rear seat back and the structure around the shock towers.
6. A few strips in the trunk bottom and side walls, but not too much.
Probably 20-30 square feet AT MOST for all of this. (it goes fast) Which will add like 6 pounds to the car.
This should get you 90% of the benefit of a full application with minimal cost and added weight.
If you are brave you can take out the headliner and the dash to get to the firewall. An X or >> pattern in the roof would be enough. Adding two, 1", strips in an X or // pattern under the carpet in each footwell won't hurt either.
If you think you can quiet it down more, you can always add more later. But there is no sense in plastering the whole car with $200 of dynamatt if $50 will get you 95% of the benefit.
These guys have the products I'm talking about, but I have no affiliation with them, nor have I purchased from them.
Sorry, I don't have pics, but I've done this on 3 cars so far with outstanding results.