ResoNix Guide to Sound Treating a Vehicle

Unfortunately, there is a lot of misinformation and marketing fluff out there when looking for information on properly sound deadening your vehicle. I’m writing this to clear up any misconceptions about installing any sound treatment to your vehicle, as well as providing an easy to follow, step by step guide that doesn’t misguide you for one reason or the other.

For starters, there are 3 main categories in sound treatment for your vehicle. The first one is to control structure-borne vibration, also known as resonance. Constrained Layer Dampers, commonly referred to as CLD are the most common types of product to handle this job. The second category is to decouple panels and to prevent them from vibrating against each other. A typical product for this is Closed Cell Foam, commonly referred to as CCF. The last main aspect of sound treatment for your car is blocking outside noise. For this, you need something with mass. The most popular product is Mass Loaded Vinyl, commonly referred to as MLV.

Controlling Resonance With Constrained Layer Damping

Let’s start with the most basic, common, and in my opinion, essential forms of sound treatment for a car audio system, a Constrained Layer Damper (aka, CLD). A CLD is a product that features a viscoelastic layer of butyl rubber, and a thin sheet of aluminum as a constraining layer. A CLD’s job is to lower structure-borne vibration of panels by converting mechanical energy (the vibration) into heat via the viscoelastic properties of butyl while the constraining layer helps the panel keep its shape and resist deformation. The primary way that CLD achieves this is through shear forces in the butyl layer caused by being constrained between the panel and the constraining layer while the panel is vibrating (flexing). When the panel flexes, so does the CLD. When the viscoelastic layer stretches and deforms, it naturally resists and wants to “snap” back into place. See Figure 1.

Figure 1.

A CLD should be used where you want to cut down on any structure-borne vibration and resonance. Long, flat, and thin panels, like outer door skins, door panels, quarter panels, rear decks, and roof skins are the biggest offenders. Reducing structure-borne vibration helps lower the overall noise floor of the vehicle while also lowering the resonance caused by sound systems to an acceptable level. Lowering said resonance would result in a faster decay of bass and midbass frequencies. Slow decay in these frequencies can cause “muddy” sounding bass response or can make it seem like repeated rapid bass notes sound blended together into something unpleasant or inaccurate. See figure 2.

Figure 2.

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 between 0 and 300 milliseconds. This measurement was taken by putting a speaker in a sealed enclosure, and enclosing the front of the speaker 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 how to use it. Thankfully, it’s pretty straightforward. For starters, large, flat panels that have 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.

Figure 3.

The next thing to consider is that larger pieces are better than smaller pieces, EVEN when they total to the same amount of coverage. The third thing to consider is that diminishing returns come in at around 25%-35% coverage, although the “stage 1” sound deadening package that I offer at the shop still includes up to 50%+ coverage on the outer door skins. More is always better, but diminishing returns do exist. The fourth thing to consider is that multiple layers are not useful. You’re better off getting a better CLD and using less of it, vs. a cheaper one and doing more work getting more coverage, or even wasting time/materials on applying multiple layers. Layering up will result in some additional damping properties, but not nearly as much as the first layer. That’s because the second layer is only acting as a mass loader (which is an extremely inefficient way of damping and only lowers the resonant frequency of the panel) and is only acting as a constraining layer to the first layer’s aluminum constraining layer. Long story short, don’t waste your time or money.

So, what makes a Constrained Layer Damper good for the task at hand? Well, there are a few things to consider, but again, there are many misunderstandings. The main one is that a thicker, or heavier product is what makes a good CLD a good CLD. This is NOT the case and something I will touch on later. First, I want to focus on one of the worst bits of misinformation that floats around the internet, and that is that roofing products from your local big box hardware chain are a good substitute to a CLD. This is absolutely NOT the case. Let’s start with that last point first. Roofing products, or any product that uses any sort of asphalt should not be considered. A few reasons. The main one is that asphalt has no meaningful viscoelastic properties. This means that it will do pretty much nothing in terms of damping via a viscoelastic layer. Figure 4 is a before and after measurement done by a hobbyist on the DIYMA forums. Credit for this goes to TOOSTUBBORN2FAIL.

Figure 4.

As you can see, the amplitude or decay of the resonance does not lower at all, it just slightly shifts to a lower frequency. The reason for this is its acting as a mass loader instead of a constrained layer damper. Mass loading is another way of damping structure-borne vibration, but it is exceptionally inefficient vs. a traditional CLD. Instead of lowering resonance by preventing the panel from flexing, all it does is add weight which in turn lowers its resonant frequency. Lowering the resonant frequency may, or may not move the peak of the resonance outside of the frequency range you need in something like a door. In my opinion, attempting to mass load a panel to dampen resonance is lost cause.

To compare the above results of a proper CLD from the same test, here in Figure 5, are the results on another brands CLD that no longer exists.

Figure 5

As you can see, the peak in the frequency response greatly reduced and the waterfall graph shows a much quicker decay around the panel’s resonance, which is about 90hz in this case.

The other problem with using a hardware store bought look-a-like product and its asphalt-based adhesive is resistance, or lack-there-of, to heat. If you do a quick google search, you will find countless accounts of people who were given bad advice about using these types of products who have had them melt off of their doors, roofs, trunk lids, etc. and cause a sticky mess that is not only impossible to clean up, but also smells like tar. Occasionally you see people that say “but I used it in my car and it works great!”. Well, I’d be willing to bet two things off of that statement. The first is that I guarantee that they have never used a quality butyl-based constrained layer damper before. The second bet is that its not actually reducing resonance, but just acting like a not-so-great decoupler and reducing some panel on panel vibration.

Here are some examples of what can happen when you use a cheap sound deadening product with an asphalt-based adhesive layer or even a product that has a low quality butyl based adhesive. The following photos were sent to me voluntarily to use in this article.

As you can see, saving a few bucks now can end up cost you more money and frustration in the long run. Its better and cheaper to just use a high-quality constrained layer damper with a methodical approach right from the start.

Back to real constraining layers… There are still plenty of differences that are unfortunately largely ignored and are marketed in ways that are silly. Again, a thicker product doesn’t mean it’s better than another. A heavier product, as mentioned before, does not mean its better than another. And a thicker constraining layer does not mean it is better than another. All testing shows that the butyl formula used in the product is the most important aspect of what makes a good CLD, a good CLD. Unfortunately, saying “we use 123 Butyl with XYZ additives” doesn’t translate to anyone who doesn’t have a PhD in chemical or mechanical engineering. Another unfortunate set of facts is that the standard measurement system for these types of products is hardly ever advertised, and even if they were, they don’t 100% correlate with our application in my opinion. SEA J1637 is a composite loss factor test that is an objective measurement for exactly what we are looking for, but unfortunately, they are almost always done at 200hz, which is above the frequency range we are usually concerned about. In most cases, we are more concerned with 100hz and under since that is where a doors (and most other panels that we treat) resonant frequency lies as well as the most demanding frequencies that our midbass drivers will play.

Key Notes:

. CLD is only for lowering structure-borne vibration, aka resonance

. CLD isnt efficient at blocking road noise. A noise barrier should be used for this instead (see below)

. CLD can lower overall noise inside of the vehicles cabin due to lowering resonance of panels. How much depends on the car. Usually noticeable, but nothing crazy

. A CLD will likely not lower panel-on-panel vibration if used alone (see decoupling below)

. Using multiple layers is a waste of time, effort, and money

. Roofing products from big box stores are not a suitable solution

. Using more of a cheaper product may cost more in the end to achieve X results.

. Beware of products that come in rolls or as a folded mat. Bending the material compromises the viscoelastic properties of the butyl layer and lowers its performance.

. Just because 2 different CLD’s have the same thickness in their butyl and/or aluminum layers does NOT mean they will perform the same. The butyl formula is the most important factor to a quality CLD

Decoupling

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 compliance and thickness of the foam is what is going to 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. 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 soon to be released ResoNix Rope, which is a butyl rope, and ResoNix Fiber Mat.

Key notes:

. Is only meant to prevent panels from vibrating against each other

. Will not reduce road noise by any amount, period and should not be used to attempt to block outside noise from entering the cabin

Blocking Outside Noise

The final piece to the puzzle after you have lowered structure borne resonance and eliminated all of the panel-on-panel vibrations is to lower the outside noise entering your vehicle. This is what I refer to as soundproofing your vehicle. This is the most daunting and time-consuming task when sound treating a vehicle. There are two ways to eliminate sound from entering an area; blocking and absorption. Blocking is going to be the most effective way for a car 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, in order to block noise, you need to have a decoupled mass. The most popular choice for this in the aftermarket car audio world is Mass Loaded Vinyl, aka MLV. Typically, 1/8” thick, 1 pound per square foot virgin MLV. Stay away from any recycled MLV as it tends to have a not so pleasant odor. Applying MLV to sound-proof your vehicle, while a daunting task, is relatively straight forward… 100% coverage is needed if you want to make your efforts worth it. Sound will follow the path of least resistance and it will find its way into your vehicle if you don’t do a good job making sure you have 100% coverage with no gaps. If you are going to apply MLV to your floor, don’t waste your time unless you are able to cover every little spot. It’s best to do the whole car if you are going to attempt soundproofing in my opinion. Floor, trunk, doors, etc. When I tell this to people they usually question if only doing most of the certain areas will be enough. In my opinion, it won’t 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 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 morning 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, the 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 soundproof your vehicle. Take this into consideration. That said, ResoNix will most likely not be offering any noise barriers such as mass loaded vinyl. Frankly, there is no point. Virgin 1lb/sq. ft MLV is exactly what it is. The prices you will be able to find on Amazon for the same product will be lower and include free shipping if you have prime. We just will not be able to compete so I am not going to waste my time. We MAY have a noise barrier of some sort in the future, but it will not be MLV unless I just so happen to stumble upon a manufacturer giving it away for free.

Considering all of that, we will soon be offering a product designed for absorption/decoupling. If you have ever worked on some newer, nicer vehicles you may have seen a fiber mat type material in the door panels. A good amount of car manufacturers use these in their doors, but I have never seen it offered in the aftermarket.

I always wondered why too, since it kills two birds with one stone; absorption of higher frequency noise while also acting as a decoupler. While it may not be as good as a perfect closed cell foam, it is still a better decoupler than most other CCF’s on the market. Absorption is a little more difficult to predict than blocking. For absorption, there’s more to it than just full coverage with as much mass as possible. Different absorbers have different fiber sizes, densities, thicknesses, etc. Long story short, an absorber should have very fine fibers or open cells that essentially restrict sound waves and convert acoustic energy into thermal energy. While it may not do as much as a barrier like MLV, it can still help a little bit with the higher frequency harmonics of road noise as well as wind noise. Based off of generalized calculations, I’d say our ResoNix Fiber Mat can help with unwanted noise that resides at 800hz and above. We suggest using our Fiber Mat on door panels, rear decks, behind headliners, behind A, B, and C pillars, etc. It is extremely compressible and easy to work with and will fit pretty much wherever you would like it to.

Key Notes:

. Do the entire vehicle with as close to 100% coverage with no gaps to the best of your ability if you want to lower the perceived volume of road noise

. Attempting to reduce road noise by only doing a few areas of the car (like just the doors) is a waste of time, effort, and money

. Having an air gap between the noise barrier and the panel of your vehicle is important. It is not suggested to use a noise barrier without a decoupling layer between the car and said barrier

Installing it: Doors

Taking all of this into account, let’s start installing it! When building a car audio system, one of the first things you’re probably going to be doing is swapping out your stock speakers for a better sounding, more robust set and in most cases installing the woofer in the OEM location in the lower corner of the door. If this is the case, the doors are most likely going to be the first area you will apply any form of sound treatment materials to your vehicle. When working on the door, there are 3 separate areas to focus on; the outer door skin, which is the outer most panel of the door. The outer skin is usually going to be the most essential part of the door to attack resonance as they are typically large, flat, and highly resonant. Looking at an outer door skin from the inside you will usually see some crash bars. It’s a good idea to use some ResoNix Rope (coming soon) in between the crash bars and outer door skin to start. The crash bars will act as a constrained layer damper to the outer door skin in this situation. If your vehicle has a foam adhesive connecting the crash bars to the outer door skin, use butyl where you can. The foam adhesive alone isn’t doing much.

After the gaps between the outer skin and the crash bars are filled with butyl rope, you want to apply a healthy amount of CLD Squares to the outer skins. Use the largest pieces you can and as mentioned earlier, start from the center and work your way out. Make sure the surface is clean (use rubbing alcohol or wax & grease remover and a clean cloth or paper towel to get rid of all dirt and grime) and use a roller to ensure proper adhesion of the CLD to the panel. It’s a good idea to put a bit of extra focus into the area close to the speaker. Other than that, start from the center and work your way out.

Door panel removed and the OEM fiber mat and moisture barrier removed, and wire harness tucked out of the way. Here you can see the outer door skin exposed, along with the two support bars.

Here you can see how we used butyl rope in between the crash bar and outer door skin in this 2018 Honda Accord front door.

Here you can see the cars OEM damping material. It is usually not worth the time and effort to remove it. Either leave it as is and work around it, or dampen over it.

A healthy amount of ResoNix Squares was applied to the outer skin.

After the outer door skin is finished, let’s move onto the inner door skin. The inner door skins can be a bit unpredictable from vehicle to vehicle. Outer door skins are pretty much always long and flat. Inner door skins, not the case. Some are flat and easy to work with, some have a bunch of curves and bends in the sheet metal, some have wires all over them, and most have large holes in them as well. Some are even made out of plastic or composite materials. If they are large and flat with not much natural damping, stick to the ResoNix Squares. If not, some more careful thought and planning should go into it. If the panel is already naturally damped (knock on it with your finger. Does it resonate or is it solid?) If it already has some form of natural damping due to its shape and curvature, stick to ResoNix Rectangles unless you are after the absolute best performance regardless of price. Just know that diminishing returns do exist. Again, use the largest pieces you can. It’s better to have one large piece as opposed to multiple smaller pieces that cover equal, or even more surface area. The largest problem with inner door skins is the large holes that are there to service anything inside of the door. For optimal midbass performance, we need to do our best job at sealing these holes to prevent cancellation from the rear wave of the speaker interacting with the front. If the holes are large (over 1 square foot or so), its probably best to use something hard like acrylic, fiberglass, abs, etc. to seal up the holes and use a CLD over them. If the hole is on the small side, it is perfectly fine to use a CLD over the hole to seal it.

The two large holes were sealed with 3/16” ABS plastic.

After they were sealed, they were fully covered with ResoNix Rectangles.

After treating the inner door skin with CLD, it’s time to apply our decoupler. We plan on offering two options for this. One is a closed cell foam, the other being a hydrophobic fiber mat. The closed cell foam will be a little better at decoupling if done correctly, but the fiber mat has two advantages. It kills two birds with one stone. It also acts as an absorber for high frequency noise, and you won’t need to be as meticulous when applying vs. CCF. Let’s start with the one basic rule of decoupling: 100% coverage. You will want total coverage of areas that are prone to panel on panel vibration. Doors, rear decks, trunk panels, etc. There are tons of spots that have the potential to buzz and rattle so its best to attack an area as a whole. With closed cell foam or any other decouplers, you can also spot-treat individual panels and clips. Door panels, rear decks, and pillar panels, for example, have multiple overlapping layers, clips, and other pieces that snap into place. Being meticulous with all of these areas will be beneficial in cutting down on rattles.

I personally usually split the door into 3 sections and hang the CCF sheet by adhering only a small strip across the top. While it hangs, I use a marker to draw out what needs to be cut out and removed.

100% coverage with CCF.

Next up is taking care of the door panels. This part is probably the trickiest, especially if you have door mounted midbass drivers. Every door panel is usually drastically different than the next, so it’s hard to give step by step instructions on what to do. What I like to do is break it down like I’m deadening a car. Are there any large flat surfaces? If so, scuff them up with rough sandpaper, clean it, and apply CLD. I highly suggest ResoNix Squares for the door panel, and remember, small pieces aren’t going to do much. Try to use larger pieces if possible. After you have targeted any areas that might be prone to resonance, focus on decoupling. As mentioned previously, door panels can be made up of multiple different layers and pieces. If these layers are easily separated and you can hear then vibrate against each other when you knock on the panel, you will want to spot-treat the area where they meet with closed cell foam or even butyl rope. It’s also a good idea to hold door panel clips into place with tesa tape or butyl rope to prevent them from vibrating against their housing.

Below, you can see a door panel that we did for a 2014 Mazda CX5. Not only did we use a CLD on the large, flat surfaces of the panel, we also used Tesa tape, CCF, and Butyl Rope to decouple various parts of the panel from one another to prevent any audible buzzes or vibrations. If you zoom in on the second picture, you can see some of the areas that we treated circled in red.

Close-ups of Tesa Tape and Butyl Rope being used to prevent the clips from vibrating.

This just about wraps it up for our method of getting the most out of sound treating your doors. The only other thing you could add is a noise barrier, but we don’t see a point in doing that unless you are soundproofing your entire vehicle. Unfortunately, we do not sell a noise barrier so we do not see a need to include it in our guide. Long story short, it would go between your door car and the decoupling layer. Ideally, it would have a decoupling layer on both sides.

Stay tuned, we will have more guides for treating various areas of a vehicle up shortly.

If you feel we need to clarify on or rearrange anything in our guide, please feel free to send us an email and we can further expand. Thank you.