The top two graphs pictured above is what’s called a waterfall graph. Waterfall graphs are much like frequency response graphs but include another domain on top of frequency vs. amplitude, which is time. Along the X-axis, we have frequency in Hz., and Along the Y-axis, we have amplitude in decibels. Along the Z-axis is time. In this case, the Z-axis is from 0 to 300 milliseconds. We took this measurement by putting a speaker in a sealed enclosure and enclosing the speaker’s front as well, with one panel being a 12″ x 12″ piece of 16-gauge steel. The microphone was placed at the center of the panel and 1/8″ away.
The initial frequency response is in the back of the Z-axis, and as you move forward, it shows the decay of the response vs. time. The quicker various frequencies decay, the better the panel is damped. As you can see, when we added a 6″ x 4″ piece of ResoNix Square to the center of the panel, the resonance was very well damped.
Now that we covered what a constrained layer damper is (CLD) and what it does let’s talk about using it properly. Thankfully, it’s pretty straightforward. For starters, large, flat panels with no natural structure or damping to them (think outer door skins, roof skins, trunks, quarter panels, etc.) are what you want to focus on first. When applying a CLD to panels, it’s best to start in the center and work your way out. See Figure 3.
NOTE: This last part is only the case for general soundproofing. For sound system upgrades, starting near/behind the speaker is your best option.
The next and second most crucial aspect category in sound treatment for your vehicle is also pretty straightforward; a decoupler. Closed Cell Foam, aka CCF, is the go-to type of product for this job. The primary goal here is to provide a soft cushion between two panels using the natural compliance of the foam to prevent them from vibrating against each other, creating audible buzzes and other annoyances. The foam’s compliance and thickness will separate an ideal foam from a not-so-ideal foam in different situations. I prefer to use a foam that’s as thick as possible without compromising the integrity of the re-installation of the panels, but also has high compliance without being too weak like open-cell foam. To go into more detail, the properties that need are used to pick an appropriate decoupling material for our uses are Compression Deflection and Compression Set. Compression Deflection is a measure of the resistance of a material to force applied to a known surface area over a controlled distance. Compression Set is the amount of permanent deformation that occurs when a material is compressed to a specific deformation, for a specified time, at a specific temperature. The standard testing for these is ASTM D 1056. These reasons are why I chose the exact foam and thickness for ResoNix Closed Cell Foam. When applying closed-cell foam, it’s best to use 100% coverage while also spot treating those little nooks and crannies where two panels can meet. Another excellent product for those tough-to-reach spots like seams in door panels is our ResoNix Rope, a butyl rope. We will soon be releasing another decoupler that also has high-frequency absorption properties. We will release more info on this when we can.
After you have lowered structure-born resonance and eliminated all of the panel-on-panel vibrations, the final piece to the puzzle is to reduce the outside noise entering your vehicle. This part is what I refer to as sound-proofing your vehicle. Frankly, this is the most daunting and time-consuming yet rewarding task when sound treating a car. There are two ways to eliminate sound from entering an area; blocking and absorption. Blocking will be the most effective way for a vehicle since blocking only needs mass with an air gap, while absorption needs a relatively thick open-cell or fibrous material. You would need an absorbing material that is way too thick relative to the size of the car to do anything meaningful for road noise. That said, those types of products still have their place, but let’s focus on blocking noise right now. As said before, to block noise, you need to have a limp, decoupled mass. The most popular choice for this in the aftermarket car audio world used to be Mass Loaded Vinyl, aka MLV. Typically, 1/8″ thick, 1 pound per square foot virgin MLV. The new favorite among die-hard enthusiasts is encapsulated sheet lead. Applying a noise barrier to sound-proof your vehicle used to be a daunting task. 1/8″ MLV is very stiff and has no stretch, and does not conform well to bends and curves. Our ResoNix Barrier is very flexible and holds its shape. It is also much thinner than previously prevalent noise barriers, making it the obvious choice to use in today’s vehicles. Using a noise barrier is relatively straightforward, and there are a couple of rules. For starters, 100% coverage is highly suggested. If you are going to apply a noise barrier to your floor, do not waste your time unless you can cover every area. It’s best to do the whole car if you are going to attempt sound-proofing in our experience. Floor, trunk, doors, etc. When I tell this to people, they usually question if only doing the more general areas will be enough. Again, in our experience, and while there may be exceptions to the rule, it typically will not be worth the effort. As I said before, sound will find its way in the car. Here is an analogy that I have lived through many times that I still use to explain… Say your annoying and inconsiderate next-door neighbor decides to cut his grass with his obnoxiously loud mower at 7 am on a Sunday while you’re trying to sleep in on your only day off. It just so happens that it was a warm night, and you slept with your window open. The second he fires up that old John Deer of his, it wakes you up. You think to yourself, “Ugh, here we go again. Better close my window and try to get back to sleep”. What happens to your perceived volume of his mower when you close your window halfway? Nothing, right? What about when you close it 90% of the way? Still pretty much no different than with it fully open. What about when you close it pretty much all the way but don’t lock it and have a good seal? Yeah, perceived volume is lower, but not by as much as you had hoped. Your perceived volume of his mower only becomes significantly lower and tolerable when you fully seal that window shut and lock it. It’s no different when trying to sound-proof your vehicle. Take this into consideration. You also need to decouple using a closed-cell foam or another appropriate decoupler from the substrate (the car’s body) if you want to make your efforts worth it. Without being decoupled from the car’s body, energy will pass from the car’s metal right into the noise barrier, and sound will radiate off of the vibration of the noise barrier itself. Sound will follow the path of least resistance, and it will find its way into your vehicle if you do not do an excellent job of making sure you have 100% coverage with no gaps. ResoNix Barrier is the ideal choice for this task and has other added benefits that typical noise barriers do not have. The biggest complaint about the previous go-to, Mass Loaded Vinyl, was that it was relatively stiff, very thick, and hard to work with. Newer cars tend to have very tight tolerances behind panels, and there is just no room to fit MLV in most new vehicles. ResoNix Barrier’s middle lead layer is extremely thin at about 1/64″ thick. It is easily moldable, as if it were a very dense aluminum foil. For those who do not have the tools, materials, or means to make them out of an appropriate plastic can also use ResoNix Barrier in place of “block off plates” for the holes on the inner door skin. We will soon add pictures to demonstrate this, but it’s pretty simple. Just hang the piece over the inner door skin and cut around the shape of the door. While it is hung in place, make any necessary holes for the speaker, wires/plugs, and door locks/release cables. You can attach it to the inner skin at the top with our Velcro or even mechanical fasteners with large washers and seal around the edges with something as simple as aluminum HVAC tape or self-cut strips of our CCF.
Now, let’s talk about the elephant in the room. Yes, our product contains lead sheet. Lead, at least in this capacity, is not nearly as dangerous as one would typically think. Lead is usually only a concern when its dust is absorbed through a mucus membrane. It typically must be ingested or inhaled to be of any concern. Any of the typical warnings of handling lead are not a worry with ResoNix Barrier. This is because the entirety of the lead has our closed-cell foam encapsulating both faces. The only bits of lead you will be able to touch are on the very edge of the product. We still recommend using gloves and a dust mask when cutting our product, but we assure you that there is no worry considering how this product is handled and installed.