Updated Last: 5/5/2021
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 car, 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 four main categories for 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 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 category 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.
Let’s start with the most basic, familiar, 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.
A CLD is 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, 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 a “muddy” sounding bass response and make it seem like repeated rapid bass notes sound blended into something unpleasant or inaccurate. See 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, 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.
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 helpful. 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 a highly 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 constraining layer. Long story short, don’t waste your time or money. We will have data posted for all of this soon enough.
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 product or even heavier product 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 for a CLD. I cannot stress this enough, but this is absolutely NOT the case. Let’s start with that last point first. Roofing products or any product that uses any asphalt should never 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. Just because it looks like a constrained layer damper doesn’t mean it is one. Figure 4 is a before and after measurement done by a hobbyist on the DIYMA forums. Credit for this goes to TOOSTUBBORN2FAIL.
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 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 a lost cause.
To compare the above results of a proper CLD from the same test, here are results in Figure 5 on another brand’s CLD that no longer exists.
As you can see, the peak in the frequency response significantly 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 you will come across when 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 real, butyl-based constrained layer damper before. The second bet is that it’s not actually reducing resonance but just behaving like a not-so-great decoupler (more on these below) by reducing some panel on panel vibration.
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.
Ok, back to actual constrained layer dampers. There are still plenty of differences that are unfortunately largely ignored and are marketed in silly ways. Again, a thicker product doesn’t mean it’s better than another. A heavier product, as mentioned before, does not mean it’s better than another. And a thicker constraining layer does not mean it is better than another. All objective, data-backed testing shows that the butyl formula used in the product is the most critical 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 Ph.D. in chemical or mechanical engineering and has years of experience in this exact field. Another unfortunate set of facts is that the standard measurement system for these products is hardly ever advertised. 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 precisely what we are looking for. Still, unfortunately, they are almost always done at 200hz, which is about an octave above the frequency range we are usually concerned with. In most cases, we are more concerned with 100hz and under since most vehicle doors (and most other panels that we treat) resonant frequency lies and the most demanding frequencies that our midbass drivers will play.
. CLD is only for lowering structure-borne vibration, aka resonance.
. It does *not* block road noise. While it does help lower the overall noise floor by reducing structure-borne vibration, you should use a noise barrier for blocking outside noise.
. Most likely will not lower panel-on-panel vibration if used alone. This is different than structure-borne vibration, aka resonance (see decoupling below).
. Using multiple layers is a waste of time and money. Just use one layer of the best product you can get your hands on.
. Roofing products from big box stores are not a suitable solution.
. Using more of a cheaper product may cost more and takes more time and effort 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.
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.
. It has two primary purposes; to prevent panels from vibrating against each other and to provide an air gap for a noise barrier (more on this below).
. It will not reduce road noise by any amount, period, and should not be used to attempt to block outside noise from entering the cabin.
. The ideal decoupler is very compressible but still has a firmness that can still provide some stability between panels.
After you have lowered structure-borne 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.
. 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. The headliner will not be possible in most cars due to the weight of a noise barrier, so stay tuned for our future product to handle that.
. Attempting to reduce road noise by only doing a few areas (like just the doors) is usually a waste of time, effort, and money. The significant gains come the closer you get to 100% coverage.
. 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. That air gap is typically closed-cell foam, which ResoNix Barrier already features.
. When using ResoNix Barrier, it is not 100% needed to use more foam, but it is never a bad idea to have more cushion for decoupling if there is still plenty of room.
A few details about our following upcoming product.. While it may not be as versatile as a perfect closed-cell foam, it is still a better decoupler than most closed-cell foam products on the market. On top of that, it will also act as an absorption layer for higher frequencies and is used in hard-to-reach/install areas. Absorption is a little more challenging 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 ResoNix Barrier, it can still help slightly with the higher frequency harmonics of road noise and wind noise. We will post more info as soon as we can.
This brings us to our fourth and final category of sound treatment…
Absorption is pretty simple on the surface but can be tricky at the same time. The concept is simple, but doing it in a car and making products that are considered high performance and can work without issue in an automotive environment is challenging. I’m going to try to keep this as simple as possible. First, sound absorption happens when you convert acoustic energy into another form of energy, such as mechanical or thermal. It only makes sense to use a soft, fibrous or porous absorber in a vehicle, so we will stick with how those work. The fibrous materials absorb sound by deforming randomly at a microscopic level as sound passes through. The porous materials absorb sound by creating friction against sound waves as they pass through. How much sound is absorbed comes down to a few things.
So, long story short, while keeping it a bit generalized, higher frequencies are easier to absorb, and a thicker absorber will allow you to absorb lower frequencies more effectively. Now, in a car, we have a couple of reasons to want to absorb noise. The first is obviously to lower the outside noise that can make it into the vehicle’s cabin. The second is for improvements in your sound system. Let’s start with the latter since that’s what most of you reading this are interested in. The main focus for absorption for automotive sound systems is most definitely going to be inside the doors if you have door-mounted midbass drivers. The purpose of absorption here is to do two things. Lower amounts of energy make it from the speaker’s rear wave to the outer door skin and lower the energy that makes it back to the midbass drivers’ cone; both will reduce distortion. Unfortunately, you cannot just go and stick any old acoustic foam in your doors since it will hold moisture, grow mold, and prematurely rust your doors. You need something that can absorb AND is highly water-resistant or even fully waterproof. While we do not make a product that handles this, we are lucky enough to distribute one that does: Blackhole Tiles. Blackhole Tiles are a unique product, and there is nothing else like it on the market. They managed to create a waterproof product that absorbs sound and even provides a noise barrier floating inside it. Now, most install these in a checker pattern in their doors, but if you go ahead and do full coverage, you can get some noise blocking out of them as well. We will soon update this page with a full door install that shows how to install them.
Another location to use an absorber in a vehicle is in between the headliner and the roof. Since most headliners cannot support the weight of a noise barrier, an absorber is, most of the time, all that we can use in this location. We will have a product for this soon and will add a detailed guide at our earliest convenience.
. Using any old acoustic foam in your doors is not a good idea. It will grow mold and smell once it holds water for some time.
. Due to the constraints of a vehicle, we cannot use very thick/dense absorbers; therefore, this is typically only good for reducing high-frequency noise.
Written in April 2019. Up to date info featuring how to use our new products coming soon.
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.