How To: Automotive Sound Deadening. Everything You Need To Know To Sound Treat Your 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 car, as well as provide 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 (aka CCF) or Fiber Mat. The next category of sound treatment for your car is blocking outside noise. For this, you need something with mass. ResoNix Barrier is a modern solution to a variety of problems when trying to block outside noise. Lastly, sound absorption. This is something relatively less spoken about, but still can yield significant improvements to your vehicle. Our ResoNix Fiber Mat excels at this. Keep reading to learn all about how these types of products are used and the science behind how they reduce the overall noise floor of your vehicle.
Change Log
. No records on changes kept before December 2022
. Updated December 17th, 2022 – Entire “Installation Example: Doors” section re-written & updated
. Updated February 17th, 2023 – Entire “General Synopsis” section added
Quick-jump buttons for various sections of this guide
General Synopsis Of Sound Treating A Vehicle
Before we dive into the details about how and why all of these products work, and in a way that gets more technical than anywhere else, we’re going to start with a general synopsis that should give you the basic ideas on how these products work, and when/how to use them, and when/how to not use them. This general synopsis will still be a bit more in-depth than most places, but I will try my best to keep this condensed but informative.
Step 1: Constrained Layer Dampers
Constrained Layer Dampers, which many technical hobbyists will refer to as “CLD,” and the general hobbyists and many ill-informed installers and salesmen refer to as “sound deadener,” is the most common product used to sound treat a vehicle. A CLD features two layers; a visco-elastic (fluid to a degree yet stretchy/snappy like elastic) butyl rubber layer and an aluminum constraining layer on top. Typical CLD’s use a backing paper and are peel-and-stick for their application. These products are the first to apply before any other type of sound treatment product, and they are installed directly onto metal and plastic panels of a vehicle. These panels typically include but are not limited to the doors, trunk lid, floor, rear deck, roof, various interior trim panels, etc. The point of a CLD is to lower the resonance of the panel or surface it is adhered to. It does this by using the natural shear resistance of the visco-elastic butyl stretching and wanting to snap back into place.
When the panel it is adhered to bends, the aluminum constraining layer provides a reference across the top surface of the butyl and causes the butyl to stretch and want to snap back into position. The goal when installing this product onto a panel is it will ideally go from sounding like a hollow drum when knocking on it to sounding like a wooden table top. Why is this product type an important, arguably the most essential step in the process? I’ll break it down in two ways. Car audio benefits first, and regular driving after.
For car audio purposes, when we add larger, more powerful speakers with a lot of power being applied to them, there is a lot more acoustic energy being introduced into the cabin of the vehicle, most notably in the doors and the trunk area. The doors are usually the worst offender, so let’s focus on those for the example. When you have a speaker mounted to a door, the front wave of that speaker, when installed correctly, is mostly directed through the door panel grille and into the cabin. The rest of that front wave that doesn’t make it out of that door grille due to the speakers’ dispersion pattern, that energy must go somewhere. It will end up between the door panel and the inner door skin, and that acoustic energy will transfer into both the door panel and inner skin and be converted into mechanical energy. That mechanical energy does two things. First, rattle the hell out of your door. I don’t think I need to tell you why this is undesirable. And second, it will cause resonance. Resonance will be heard as distortion and will cause the system to sound muddy and lack dynamics and impact. Applying a constrained layer damper to the door panel and inner door skin will constrain those panels, and that mechanical energy of the panels moving will then be absorbed and converted into minuscule amounts of heat by the CLD. It’s a pretty simple concept. While they do add mass, which will help lower the resonant frequency of the panel, mass-loading is NOT the main reason why or how a CLD works.
Now, how will a Constrained Layer Damper help with sound treatment of a vehicle in everyday driving with no regard to the sound system? Simple. When driving, wind and moving air and mechanical energy transferred from the tires/suspension will cause the same panels mentioned before to move, vibrate, and resonate, which will be heard as resonance, drone, and general automotive “noise.” Think of the panel acting like it’s a speaker, moving in and out to convert mechanical energy into acoustical energy. Applying a Constrained Layer Damper to these resonating panels will prevent that resonance and lower the overall noise floor while driving.
Now, something to note. Every vehicle is different, and every vehicle will behave differently. Each vehicle will require a different amount of CLD and sometimes a different approach to the overall game plan of applying a CLD, whether it be locations placed, how much is needed in certain areas, etc. I find that economy cars benefit more from these products than rugged premium and luxury cars. When we work on something like a Toyota or Honda, there is a more noticeable difference when applying a CLD vs. applying CLD to, say, a Mercedes. This is because these higher-end vehicles have much more robust and sturdier parts. That said, I have yet to find a vehicle that I have taken apart and worked on that I didn’t feel would be much better with a CLD applied, even considering diminishing returns and time spent on the project.
Constrained Layer Dampers can be applied to metal, plastic, wood, etc. Please make sure the substrate you are applying it to is clean and free of dirt, oil, and grease. I prefer to use isopropyl alcohol to clean panels before application. Do not heat the Constrained Layer Damper. Too much heat can damage the butyl layer and cause performance loss. My rule of thumb to customers with this is if it is too cold to work with, it’s too cold for you to work on your car. Go inside.
Another thing to note is that a single, continuous large piece of CLD will be much more effective than multiple smaller pieces that take up the same size. My personal rule of thumb is if the piece of CLD is going to be less than, say, 6 square inches, don’t bother unless you are using it to help with something such as a smaller rattle of an attached mechanism, trim, etc. But applying a smaller piece to a much larger panel wastes time, effort, and product.
One more thing about Constrained Layer Dampers, they do NOT block noise. That is right. Nearly EVERY other sound deadener company will claim this about their CLD product, but it is just not true. A good CLD can most definitely lower the noise floor of a vehicle since it will reduce the amount of noise generated by the resonance of panels, but it will NOT block outside noise from entering the vehicle. Any company claiming this about their product is lying to you.
When applying, make sure you roll it down after sticking it on. You do not need to press down very hard. Just enough to ensure the whole piece has contact with the panel on a nearly microscopic level is fine. Using too much force can deform the butyl, stretch the aluminum, and cause performance losses. That brings me to one more point: flat pieces that haven’t been rolled, creased, deformed, etc., will have noticeably better performance than their deformed, beaten, and mangled counterparts. This performance drop goes for products that already come in rolls as well. CLD products are manufactured in flat sheets. Rolling them up reduces their performance through the deformation of the butyl before their product even leaves their warehouse. ResoNix CLD Squares is our CLD product, and to date, they have not only tested independently as the highest performing product on the market, but the overall best value available as well.
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Step 2: Decouplers
Decouplers are the second product that will get installed in almost every traditional automotive sound treatment application. A decoupler, typically a closed-cell foam or a fibrous mat material, is used to accomplish two different goals. First, it acts as a cushion between two panels that could otherwise vibrate together and cause annoying buzzes and rattle sounds. Second, to act as an air gap and isolator for a Noise Barrier, but we will touch on that second part later, in the Noise Barrier section.
As mentioned, Decouplers are almost always a Closed Cell Foam, aka a “CCF,” but are sometimes, as in our case with our Fiber Mat products, a fibrous mat material. As mentioned above, a Decouplers typical use case is to provide a cushion between two panels to prevent them from rattling against each other. What makes one decoupler more ideal than another? Well, in my experience, being thick enough to touch as much of both panels as possible, yet still have enough compliance and is soft enough to compress down and not cause issues when re-installing these panels. Having a specific Compression Set specification (spec that tells you how well it holds its shape once compressed) can also help provide a Decoupler that can compress a lot of itself down but still provide some resistance for the panel instead of acting like a soft sponge.
Speaking of sponges, we do not want to use open-cell foam as a Decoupler. Generic open-cell foam products will hold and retain moisture and cause mold and mildew to grow in your car. I see some people who try using polyurethane foam (ordinary acoustic foam) for this in an effort to “beat the system” and save money. This generic acoustic foam, shredded denim, jute, etc., will all hold moisture and cause issues.
Another popular product among those that want to save money is using gym and yoga mats. The problem here is that the foams that these are made of defeat the purpose of a decoupler. They are too hard and stiff to decouple properly, will not effectively reduce panel vibration, and will certainly not provide correct isolation and an air gap for a Noise Barrier. A popular choice in these products that budget-conscious hobbyists use has the specifications for the foam that it’s comprised of listed on the website. It has a Compression Deflection (stiffness) spec of 18-26psi. This means it takes between at least 18 and up to 26 pounds per square inch to compress the foam by 25%. That is exceptionally stiff for our uses. Our CCF decoupler 7, for comparison, has a Compression Deflection spec of 1psi maximum, so it is 18+ times less stiff than a popular DIY option. As with all of our products, buy once, cry once.
How to install a Decoupler is simple. Most of these products, ours included, feature a peel-and-stick adhesive; you can cut it to shape and stick it where needed. Our CCF Decoupler 7 and CCF Decoupler 3 will typically be used on plastic trim, or on the metal where plastic trim will touch. Anywhere a trim panel can vibrate against another panel, this is where you want to apply a decoupler. On larger panel decoupler installations, I usually prefer to adhere the decoupler directly to the trim panel that is getting installed into the car instead of directly on the vehicle itself. I find this works better and also has much better serviceability and longevity.
What closed-cell foam decouplers do not do, even though many other companies claim this, is reduce, absorb, and/or block noise. It does NONE of those in any meaningful way, shape, or form. Beware of companies that advertise their CCF products as being able to do this. They are lying and preying on the ignorant. A quick summary of why they do not block or absorb noise is that you need a lot of mass to block noise, and you need open cells or a fibrous mat to absorb sound. A lightweight closed-cell product has no chance of blocking noise or absorbing noise. Also, beware of products from companies that claim that their decouplers block heat or are excellent thermal insulators. While many thermal insulation products are comprised of a closed-cell foam construction, it is usually a VERY different type of closed-cell foam that is used for decoupling. The foams that are used for decoupling are generally not great thermal insulators.
As hinted above, we offer a few options for Decoupling. Our most obvious one is our CCF Decoupler 7, a closed-cell foam product that is 6.5mm thick. This is best to use for smaller panels and tight spaces. Our soon-to-be-released CCF Decoupler 3, a 3mm thick foam, will be even thinner and for even more fine detail work. Small clips, switches, etc., that might rattle can be handled with CCF3.
Meanwhile, our Fiber Mat 25 and Fiber Mat 45 are best used for larger areas. Think of door panels, rear deck trim, quarter panels, pillars, etc. The Fiber Mat products are much better at decoupling and offer an added benefit: Absorption.
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Step 3: Sound Absorbers
Absorption is next and third on the list of sound treatment products to be installed into a vehicle. Absorption is pretty simple for the most part. Acoustic absorption converts airborne noise into another form of energy, therefore reducing the overall noise level in the vehicle. An open-cell or fibrous materials are two typical types of acoustical absorption products. The thickness, density, size of the fibers or open cells, and more contribute to what frequencies are absorbed and how much they are absorbed. Higher frequencies are easier to absorb than low frequencies. However, some products can effectively absorb relatively low frequencies in a vehicle, such as our ResoNix Fiber Mat 25 & 45, and our ResoNix Guardian. Both have two different areas and reasons for application, so we will break them down individually.
ResoNix Fiber Mat 25 & 45, as mentioned earlier, are an acoustic absorption and a decoupling product. Our ResoNix Fiber Mat products are made out of microscopic synthetic melt-blown fibers that are formed into a mat, have a peel-and-stick adhesive on the back, are 100% okay with being installed in the harshest automotive environments and will not grow mold or mildew, and are naturally hydrophobic. They handle water, heat, cold, and anything else just fine, so ResoNix Fiber Mat can be installed practically anywhere without worry. They are typically used on door panels, quarter panels, rear decks, roofs, wheel wells, fender liners, pillars, sometimes even floors, trunk floors, and anywhere else you can think of.
The general rule of thumb with ResoNix Fiber Mat 25 & 45 is any application is always increasingly beneficial. Any time you add more of an absorption product, you will theoretically absorb more noise. There is one caveat, though. Stuffing it to the point of compression will alter the product’s behavior and shift and change the acoustic absorption properties, usually in a way that is negative to our requirements in the automotive environment. For larger voids such as roofs, quarter panels, etc., only fill them until they are full, but do not add extra to the point that the Fiber Mat compresses. Fiber Mat should remain “lofty” for the best performance. The only time compressing is okay and even preferred is when you are using Fiber Mat as a decoupler as well. Light compression of ResoNix Fiber Mat will drastically increase decoupling performance and panel stabilization without having enough adverse effects on absorption to be concerned. Door panels and rear decks come to mind here since these are areas that we are typically very concerned about with panel-on-panel vibration.
Regarding picking Fiber Mat 25 or Fiber Mat 45, that’s hard to say. While we have done many vehicles, we still haven’t done most. While we can help guide you based on our experience, it is ultimately up to you to determine which version best suits your application. Remember, fill, but do not compress. So far, in my experience Fiber Mat 45 will be better suited for larger tolerance door panels, rear decks, rear hatch trim panels, quarter panels, roofs, fender liners, etc. Fiber Mat 25 is typically suited for smaller tolerance door panels, pillars, floors, trunk floors, tire wells, etc.
Installing it is very simple. Just cut to shape with heavy-duty scissors (I use 10″ and 12″ Wiss scissors), peel the backing paper, and apply it to the desired surface. As with anything, ensure the surface is clean and clear of dirt, oil, and grease. I prefer to clean the panels with isopropyl alcohol beforehand. You can stick this anywhere you want, so long as it is clear of any moving parts. I do not recommend installing these inside door cavities as an absorber as I worry that window mechanisms can snag the material and cause issues. Same with seat belt areas. But everywhere else, free game.
ResoNix Guardian. This new product from ResoNix Sound Solutions is a familiar concept to some that has been further improved. It is a composite-style material that consists of ¼” Hydrophobic Melamine foam, 1/8″ thick 1lb Mass Loaded Vinyl (aka MLV), another layer of 1″ Hydrophobic Melamine, and an Acoustic Polyester Facing. It features a peel-and-stick adhesive for easy installation. This product is multi-faceted, so we will give a general breakdown of each.
First, it is used inside doors, subwoofer enclosures, or anywhere behind speakers that produce a lot of energy. The point here is to absorb the acoustic energy inside the door or enclosure BEFORE that acoustic energy makes it to the door skin or enclosure wall. Since ResoNix Guardian’s Hydrophobic Melamine layer absorbs noise, and the Mass Loaded Vinyl layer deflects noise, all before said noise reaches the door skin or enclosure wall, the amount of acoustic energy that even reaches those boundaries is significantly lessened, therefore reducing the overall amount of resonance and vibration that is created.
The second use for ResoNix Guardian is to make use of the floating layer of 1/8″ thick, 1 pound per square foot Mass Loaded Vinyl Layer. We didn’t cover noise barriers yet, but this is the first product that makes it extremely easy to install a noise barrier to the roof and sides of your vehicle. Previously, installing a noise barrier on vertical, or god forbid overhanging surfaces, was difficult, tedious, and not consistently effective. Mechanical fasteners, specific Velcro (that we still offer), or other hoops must be jumped through to install noise barriers, such as Mass Loaded Vinyl or ResoNix Barrier, onto these surfaces. Now, it’s just peel, stick, done. And all with the added benefit of being fully decoupled and isolated, and with the help of acoustic absorption of the Hydrophobic Melamine.
The third use for ResoNix Guardian is for its thermal insulation properties. While Hydrophobic Melamine is an excellent acoustic absorber, it is also a top-tier thermal insulator, so much so that it is even used on the tips of rockets to protect them from heat. ResoNix Guardian is the ultimate solution for treating a camper van or RV since you get not only the acoustic benefits but also the thermal insulation benefits as well. It will help keep heat in when it’s cold and help prevent heat transfer into the vehicle when it is hot outside.
How it is installed is simple. Just cut it to size carefully with a sharp box cutter, peel the backing paper, apply it to the panel, and apply pressure. Again, ensure the surface is clean and clear of dirt, oil, and grease. I prefer to clean the panels with isopropyl alcohol beforehand. ResoNix Guardian is most commonly installed on top of CLD on the outer skin of doors inside of the door cavity, inside of subwoofer enclosures and speaker cabinets lining the walls, on roofs, and covering van/RV interiors after CLD application. This product is thick and not very compliant, so this is not to be used directly behind typical interior panels, in wheel wells, etc., unless there is plenty of room and there is not too much curvature. If you are planning on making use of ResoNix Guardian as a noise barrier, make sure you have FULL coverage, and make sure you butt up the edges of each piece as closely and perfectly as you possibly can, as even small flanking paths can ruin the perceived performance of a noise barrier, which brings us to our fourth category…
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Step 4: Noise Barriers
The fourth and final official step and category of sound treatment application is noise barriers. While they can be hugely beneficial and arguably the most effective wide-band sound treatment you can apply when installed correctly, they are almost always difficult to install, very picky about the installation, and can be costly. The point of a noise barrier and how they work is simple. A noise barrier is made from a material to act as a limp and decoupled mass, and said mass is used to block, reflect, deflect, whatever you prefer to call it, acoustic energy away from the area you are shielding with it. The only driving factors in the performance of a noise barrier product are its weight, and how non-resonant it is. The heavier the noise barrier is and the less inert it is, the more noise it will block. What makes them very picky is the fact that a noise barrier ideally needs FULL coverage to be effective. While not having full coverage can still yield some improvement, it will be minuscule compared to full coverage. When you do not have full coverage, sound acoustic energy will travel around and take the path of least resistance right past the noise barrier. Written in our more detailed section, I use this analogy…
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. It’s no different when trying to sound-proof your vehicle.
Another part that makes noise barriers picky and selective is that they need full coverage and a decoupling layer, ideally on both sides, to isolate them from the vehicle. Noise barriers need to be decoupled from the body of the vehicle and the panels that cover them. Otherwise, the resonance of the body panels will be transferred directly to the noise barrier, which will also resonate and radiate its own noise, rendering the noise barrier mostly ineffective.
The most popular type of noise barrier is Mass Loaded Vinyl, aka MLV. It is popular because it is heavy, limp, and, most importantly, cheap. The downsides are how thick it needs to be to achieve the desired weight and its poor flexibility and moldability. Most MLV is 1/8″ thick and weighs 1 pound per square foot. Couple that with a decoupling layer on both sides, and you have an extremely thick product that is not moldable to complex contours. In short, you end up with a product that will be extremely difficult to work with and fit behind the panels of almost all newer and even many older vehicles.
ResoNix’s solution to this is our ResoNix Barrier, which is comprised of 1/32″ sheet lead encapsulated in our 3mm thick CCF Decoupler 3. This gives us a noise barrier that is much thinner yet twice as heavy per square foot as the standard 1/8″ thick MLV, is much more flexible, and is moldable to complex contours. Long story short, ResoNix Barrier is a much higher-performing noise barrier and is much easier to work with. The downside, it is admittedly not nearly as inexpensive as MLV.
ResoNix Barrier is very easy to install. For floors, just remove the carpet, roll it out, contour it to dips, peaks, bends, etc., as tightly as you can, overlap layers and even fold them once over to lock them together to prevent sound from leaking through, and let gravity hold them in place. For vertical surfaces, I prefer to use riv-nuts and stainless steel machine screws with fender washers to hold them into place. I do not suggest using Velcro to hold up ResoNix Barrier. The foam will not be able to bear the weight and will rip. The adhesive will also tear the foam apart if you try to remove it. For roofs, this isn’t easy. You can again use mechanical fasteners into the roof braces or can somehow install it directly onto the headliner, but I am not sure how that will hold up as it may be too heavy for a headliner to support. This is where ResoNix Guardian or ResoNix Fiber Mat comes in, making treating the roof much more manageable.
ResoNix Barrier can also be used to seal access holes on inner door skins to turn the door into a quasi-sealed enclosure. Typically, sturdy plastic is used for this, but in many new cars, these inner door skins are more than just a flat opening. They are 3-dimensional and have complex curves and contours that can make it difficult to mold a plastic sheet to do this job, especially with the tighter tolerances of many newer vehicle door panels.
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Extra Random Tidbits Of Useful Products & Information
ResoNix Strips
ResoNix CCF Strips are a product that is used to couple your car speakers to the panel and grille that reside in front of them. The purpose is to direct all acoustic energy created by the speaker through the grille into the listening space instead of losing acoustic energy to the open areas behind your door panels and dash panels. This has two main benefits. First, we gain more output since we can force all acoustic energy into the listening space. The second is less resonance and distortion created by the panels themselves since the acoustic energy is no longer getting trapped behind and exciting the panel. They also provide minor stability for the panel they are coupling to. Long story short, install these around your speakers, especially midbass speakers for a free lunch of more volume, lower distortion, and fewer rattles and resonance.
ResoNix Butyl Rope
ResoNix Butyl Rope is precisely what it sounds like. It is a 3/8″ diameter butyl rope used for various sound treatments in your vehicle. Two popular uses include stuffing it between the crash bar and outer door skin to help further stabilize the outer skin, and using it in small areas where two layers of a panel may overlap and introduce the possibility of them rattling together. It can also be used on the base of clips to prevent them from rattling. See the product page for installation examples.
OEM Audi/VW Interior Non-Woven Tape
This “cloth tape” is commonly used in installations for various reasons. But for sound treatment purposes, you can use it to bundle wire harnesses that reside behind panels to prevent them from rattling and buzzing, or you can use them on clips and/or clip openings to help reduce their vibrations. The difference between the tape we sell and the popular option, ours will not turn sticky and gooey over time. See the product page for installation examples.
I hope this covers all relevant types of automotive sound treatment products and does so with enough information to understand the basic concepts of how these products work, how to use and install them effectively, and sheds light on false advertisements of other products in their categories. I also hope you continue reading the rest of our original Reference & Information Guide, which will cover the same topics and may even repeat much of the information above but will provide even more detailed information unlike anything seen on any other website.
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If this guide still leaves you with questions, or if you find it confusing in any way, PLEASE give us a call. I would love to help you one-on-one and help clean this up to be as clear and concise as possible. Thanks!
Now, onto the previously written, more technical guide.
Controlling Resonance With Constrained Layer Damping
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 “Sound Deadener”. 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 butyl layer and the panel keep their 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. As of today, ResoNix CLD Squares are not only the most effective CLD on the market, but also get you the most performance per dollar spend.
See Figure 1.
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 door skins, door panels, quarter panels, and roof skins, are the biggest offenders, but in some applications, such as higher-end car audio systems, CLD can benefit the overall end result by being used on virtually any panel inside of the vehicle. 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 and more accurate representation of bass and midbass frequencies, and lower other audible rattles and resonances that many interiors can present to a car audio system.
Slow decay in the lower 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.
Figure 2
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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 and resonance was controlled.
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, door panels, 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.
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. This is due to the second layer 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 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 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.
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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. traditional Constrained Layer Damping. 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.
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Figure 5
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.
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.
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.
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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 these types of products. Still, unfortunately, they are done at 200hz, which is about an octave above the frequency range we are usually concerned with, and done on a thin aluminum strip. In most cases, we are more concerned with 100hz and under on larger panels 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.
ResoNix vs. Popular Competitors
Note: This testing was not done by or paid for by ResoNix Sound Solutions. This was done by a very dedicated hobbyist with no relation to any company whose product results are depicted below, including ResoNix Sound Solutions. More detailed explanations of this data can be found on our Independent Testing Data page.
First, here is all of the frequency response reduction data from the test. The long story short is ResoNix is 4x more effective than the next best product. ResoNix can be seen as a light-blue trace. The Red trace is the reference of the panel with no CLD applied.
Up next, we have all of the waterfall graphs that will show frequency response vs time. This is the true test and shows how well and how quickly a product will damp and fully kill the resonance of a given panel. First, the photos, and then a GIF of all of the results. Click to view full size.
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Installation Examples
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Key Notes:
- 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.
- Many smaller pieces of CLD are not as effective as a single piece of CLD with a similar overall size. Our typical smallest size piece we bother with is at least 10 square inches.
- 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, and many times will still not yield the same result of a better product that is carefully and properly installed.
- 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.
Decoupling
The next and second most crucial aspect category in sound treatment for your vehicle is also very straightforward; a decoupler. Closed Cell Foam, aka CCF such as our ResoNix CCF Decoupler 7, is the typical go-to type of product for this job, but there are others, such as our ResoNix Fiber Mat 25 and ResoNix Fiber Mat 45 products. The primary goal here is to provide a soft cushion between two panels using the natural compliance of the decoupler to prevent the panels from vibrating against each other, which creates audible buzzes and other annoyances. The decouplers compliance and thickness will separate an ideal foam from a not-so-ideal foam in different situations. I prefer to use a foam or Fiber Mat 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 the open-cell foam that is used to absorb sound in rooms. 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 why I chose the exact foam and thickness for ResoNix CCF7 are because the foams compression deflection and compression set are ideal for our decoupling uses The Fiber Mat is even more ideal. These days, we only have CCF7 to use for spot treatment on small areas. Fiber Mat should be used on doors, rear decks, roofs, quarter panels, and any other large areas where you dont want two panels rattling against each other, with the added benefit of sound absorption, which is another subject covered below. When applying a decoupler, it’s best to get as close to 100% coverage as possible 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 product..
Installation Pictures
(Awaiting Install Photos Of Our Production Fiber Mat Products)
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Key Notes:
- 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).
- Closed Cell Foam based decouplers will not reduce road noise by any amount, period, and should not be used to attempt to block outside noise from entering the cabin. Fiber Mat style decouplers on the other hand, will absorb noise that tries to pass through. More on this below.
- The ideal decoupler is very compressible but still has a firmness that can still provide some stability between panels.
- If a closed cell foam is being advertised as a sound absorber, they are lying to you. If a foam is being advertised to block noise, they are lying to you.
Blocking Outside Noise
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 vehicle. 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 to lower noise, 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 sheet lead encapsulated in foam, which we sell as our ResoNix Barrier. 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 modern vehicles.
Using a noise barrier is relatively straightforward, and there are a couple of guidelines to follow. 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, otherwise sound will make its way in through whichever areas are not covered. This is called a “flanking path”. 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, along with the laws of physics, 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 through flanking paths that are left open. 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. 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 mechanically from the car’s metal right into the noise barrier, and sound will radiate off of the vibration of the noise barrier itself. ResoNix Barrier is the ideal choice for this task and has other added benefits that typical noise barriers do not have such as having a decoupling layer already applied to both sides, and being much thinner and flexible/easier to work with.
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 1/32″ 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 mechanical fasteners and seal around the edges with something as simple as aluminum HVAC tape or self-cut strips of our CCF.
Installation Photos
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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. 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. Sound will take the path of least resistance and enter through flanking paths.
- 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.
- ResoNix Barrier can also be used to seal large holes.
Absorption
Absorption is pretty simple on the surface but can be a bit tricky the more you dive into it. 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.
The frequency of the soundwave that you are trying to absorb – The smaller the wavelength (higher in frequency), the easier it is to absorb.
The thickness of the absorption material – The thicker it is, the longer the wavelengths (lower in frequency) you can absorb effectively.
This last one is where it gets a bit more complicated. Different materials behave differently and have different absorption coefficients. When I have the time and the data on hand, I may add a section on this.
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. Less energy makes it from the speaker’s rear wave to the outer door skin, and it lowers 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 our ResoNix Fiber Mat 25 and ResoNix Fiber Mat 45 (preferred for door panels, rear decks, roofs, etc.) products can work for this, I get nervous about window mechanisms interfering and causing issues inside of door cavities. ResoNix Guardian is a safer bet to use here. ResoNix Guardian is a unique product, and there is nothing else like it on the market. It is a fully waterproof product that absorbs sound and even provides a noise barrier floating inside it, and even provides excellent heat insulation. 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 and heat insulation out of it as well. ResoNix Guardian is our new product that is similar to Blackhole Tiles. See below for a typical Blackhole Tile installation.
Other locations to use an absorber in a vehicle is in between the headliner and the roof, door panels, quarter panels, rear decks, wheel wells, hatch and trunk floors, pillars, and more. Special note for headliners since most cannot support the weight of a traditional noise barrier product, an absorber is typically going to be the last line of defense for soundproofing in this location. Our Fiber Mat products and are perfect for this application in these locations, but our new ResoNix Guardian product is the best product that will allow for the easy installation of a noise barrier (as well as an absorber) in your headliner/roof.
Another way you can use our Fiber Mat products in your vehicle is to place them inside of, or even “stuff” empty cavities. Stuff these into open cavities such as quarter panels, wheel well areas, etc. to help further absorb unwanted noise. Just make sure there are no moving parts inside of the cavity.
A few details about our following upcoming product.. While it may not be able to fit in every location that thin closed-cell foam, such as our CCF7, can due to thickness, it is still a much better decoupler than pretty much any closed-cell foam product on the market for our uses. On top of that, it will also act as an absorption layer and can be used in door panels, head liners, and other areas where you can fit it and want to decouple and absorb noise. When it comes to reducing road noise, it may not do as much as a barrier like ResoNix Barrier, but it can still help slightly with the higher frequency harmonics of road noise and wind noise. We will post installation examples in the future.
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Key Notes:
- 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, typical acoustic absorbers are only good for reducing high-frequency noise. ResoNix Fiber Mat 25 and ResoNix Fiber Mat 45, and ResoNix Guardian all overcome this.
- Most absorption products will only be effective for mid to high frequencies. It can help lower overall noise floor, but will not take care of road noise like a noise barrier will. ResoNix Fiber Mat 45 is able to absorb effectively below 200hz, and ResoNix Guardian is also great at lower frequencies thanks to its built-in noise barrier.
- Stuffing empty cavities in the vehicle can help lower overall noise floor even further. Areas such as quarter panels, B-Pillars, etc. Just make sure it will not interfere with moving parts or see moisture.
Installing it: Doors
New Version. Written 12/17/2022
Hey everyone, Nick here. I finally have an updated door guide for you. This one being in a Lamborghini Huracan that we did at Vanguard Automotive Design. This new guide features almost everything you need to know and need to see regarding sound treating a car door. I say “almost everything” because EVERY car, every situation, and everyone’s goals are different and may require some adapting to your specific situation. The best thing you can use going into sound treating your doors, or any part of your vehicle for that matter is the information provided in this entire article, but most importantly your own common sense. We have worked on many cars, but we may not have worked on your car. Considering this, I will do my best to lay this out and explain everything as clear as possible, but your situation may have a portion that requires an additional step, or a different approach. If you feel this door installation guide, and everything else posted in this entire article do not answer all of your questions, feel free to reach out and we can help guide you, and even update the article to provide more clarity. Alright, so lets hop into it..
This 2020 Lamborghini Huracan came in with the request for a high-end sound system that could also get plenty loud, as well as a few other services, but there was one problem.. It’s a 1600hp, soon to be 2400hp track car. Since this is a track car, of course it has an ice box for the turbo system in the front trunk and a half cage that resides behind the seats in the cabin. This meant there was nowhere to install a subwoofer enclosure or amplifiers. Long story short, we got creative and came up with a solution to achieve high quality and relatively high-volume bass with good low frequency extension for this system. It was to install the Illusion Audio Carbon C10 Shallow Subwoofers in the doors. Normally, installing subwoofers in the door is a huge no-no, but there are a few details to consider in this case that made it work, and work well at that. First, the extremely low distortion of these subwoofers along with their low moving mass and low inductance allowed these subwoofers to not only play their typical 20-80hz range, but also extend up to 300hz no problem. The Lamborghini Huracan comes with an 8″ midbass in the OEM lower door location, and were sure we could fit this 10″ shallow subwoofer to use it as a subwoofer as well as a midbass and have it acoustically mate up to the 3″ midrange in the pillars. It ended up working very well, but for now, lets focus on HOW we got this to work so well. The careful and meticulous installation of ResoNix Sound Solutions sound treatment product offerings.
General Synopsis
. Outer Door Skin (part of the door that is the outermost layer)
100% coverage with ResoNix CLD Squares. This is used to damp mechanical energy that is caused by the speaker, and regular driving. ResoNix has the best performing Constrained Layer Damper on the market and is responsible for making the great end results that we achieved out of this install possible. Once we finished with the full coverage of ResoNix CLD Squares, we topped them with full coverage of BlackHole Tiles (now replaced by ResoNix Guardian), also supplied by ResoNix. ResoNix Guardian is a 12″ x 12″ x 1.375″ sheet that is constructed out of a 1/4″ Hydrophobic Melamine foam layer, a MLV noise barrier layer, another layer of 1″ Hydrophobic Melamine foam, and a non-woven polyester acoustic facing that has a peel-and-stick adhesive and are naturally fully waterproof. The absorption properties of this product achieves a few things. First, and most important, it absorbs acoustic energy inside of the door cavity that is created by the rear wave of the speaker. Absorbing this rear wave will lower the amount of acoustic energy that can excite the door skins, and it helps reduce standing waves in the door cavity. Both help clean up the sound and improve transient response of the system. The other benefit of Guardian is it helps block and absorb any noise entering the vehicle from outside of the doors. Another small thing we did with the outer door skin was use ResoNix Rope in sections between the crash bar and the outer door skin. This helps stabilize the outer skin even further.
Note about Blackhole Tiles and ResoNix Guardian: These are NOT a replacement for a standard constrained layer damper. They are a supplement and help further enhance performance by a different means of acoustic suppression.
. Inner door skin
On the inner door skin, we were pretty lucky. On the Huracans, the inner skin is thick and has 3 dimensional curves, which helps increase rigidity. The access hole is also not too large or complex in shape. We ended up doing as much coverage within reason with the ResoNix CLD Squares, and then using ResoNix Barrier to replace the flimsy plastic access panel cover. This ResoNix Barrier is secured over the access hole using stainless machine screws and provides a thin yet rigid plate that can seal off the access hole, yet still be serviceable when needed and is also thin enough and moldable enough to fit the curves and tight tolerances.
. Door panel
The door panel is what I usually worry about most. There are so many plastic trim pieces, overlapping parts that aren’t fully secured, tight spots, flimsy material, etc etc. Thankfully, the Huracan door panels are very strong on the grand scale of door panels. We covered as much as possible in ResoNix CLD Squares, and also used ResoNix Fiber Mat 25 and Fiber Mat 45. The Fiber Mat products are exactly what they sound like. A mat that is constructed of a synthetic fibrous material. It is exactly what you see in some vehicles from the factory, but the car manufacturers usually use it sparingly, probably due to budget. The Fiber Mat product kills two birds with one stone. First, it is an acoustic absorption material. It is used to absorb outside noise when entering the vehicle, and to absorb acoustic energy behind panels. Secondly, it is an excellent decoupler. A decoupler is a product that is thick enough yet compliant enough, like a pillow, that can be placed between panels and prevent them from vibrating against each other by providing cushion between them. Many other companies only use closed cell foam for this, but to date, this is by far the best decoupling product we have used, and its great that it also acts as an absorber.
All of this combined, even after having a 10″ subwoofer in each door running on 300 watts each, there were no rattles in the doors. That speaks volumes to both the quality of the materials used, and the installation of them, no pun intended 🙂
Detailed Explanation
First things first, we removed the door panels and inspected what we were working with. When removing the door panel. We find that the inner door skin (the metal part of the door that is closest to the interior of the cabin) has a medium-sized access panel/hole, and everything else is sealed and pretty sturdy. That said, there are still plenty of areas to treat. From here, we removed the factory 8” midbass speaker to assess the door cavity and outer door skin.
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Once the speaker and access panel were removed, we were able to inspect the outer door skin and door cavity. Again, a relatively normal door cavity and outer door skin. Nothing different or unique to worry about. Just a flat outer door skin with a crash bar right across the middle, which is the case for pretty much any car we come across.
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First up with the sound treatment portion, applying ResoNix CLD Squares to the outer door skin. First things first, we cleaned all of the surfaces that would be treated with isopropyl alcohol and a rag to remove all dirt, oils, and grease. Once that is done, we can move on to installing the CLD Squares. Remember, it is VERY important to keep the pieces that we install as large as possible. We go over why in the CLD section of the Reference Information article. We ended up with about 4.5square feet of coverage (4.5 ResoNix Squares) on the outer skin. We went with full coverage to get the best performance possible and to not leave ourselves second guessing. That said, we understand there is the 25% coverage rule that floats around the internet. First, let’s make it clear where this 25% coverage general rule of thumb came from. It comes from someone who wasn’t doing car audio installations. He was doing general sound treatment for the sake of just lowering the noise floor of the vehicle. Frankly, 25% coverage is not what I personally would consider acceptable for a car audio installation that has a door-mounted midbass driver. In a general sound treatment installation to lower the noise floor of a car, sure, but not in a car audio system. Let me explain why.
As we know, a CLD’s job is to lower structure-borne resonance by constraining the substrate it is applied to and converting that mechanical energy into heat. When driving under normal conditions, the resonance of the door is purely caused by 2 things. Wind, and the mechanical transfer of energy from the cars wheels and engine into the body. That’s it. Not much resonance will be caused by this relatively speaking. Now when we add a high-powered speaker to the mix and the user wants to get great performance out of that speaker, the doors are getting sealed, and a lot of acoustic energy and pressure is now being created inside of that door and that positive and negative pressure is being applied rapidly to the door. Due to this, the inner skin, and especially the outer skin are resonating MUCH more aggressively than before.
Now, in almost any sound system, we are hoping to get a door-mounted midbass speaker to extend down to at least 80hz (accounting for the appropriate crossover as well) without any acoustic low-end roll off, distortion, or resonance/rattles. We also should take note that the resonant frequency (or FS, where an objects natural resonance lies and will resonate the most) of most outer door skins just so happens to be around 80hz. This is unfortunate considering this is the frequency range we are typically trying to recreate in this location, but most doors are going to make that very difficult due to this.
Lets assume we want to do 25% coverage in this case. We now have smaller pieces of CLD that are placed throughout the area of the outer door skin. What this will do is definitely take care of most of the higher frequency resonance as well as some of the lower frequency resonance that will be centered around the panels resonant frequency. But, with that much area left uncovered, it will still allow the panel to flex and resonate at the lower frequencies near its resonant frequency. When doing full, or near full coverage, you are constraining the entire panel, therefor limiting the panel from moving as a while.
Anyways, onto the installation of the ResoNix CLD Squares on the outer skin..
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Once the application of the ResoNix CLD Squares to the outer skin was complete, we can move on to a quick and easy yet highly effective way of further reducing resonance of the outer door skin. What I am speaking of is using ResoNix Rope to couple the outer door skin to the crash bar inside of the door. Now, yes, sometimes they are already “coupled” with another type of adhesive or material from the factory. But it is usually a very light and ineffective material and is just used to provide very minor support. Using ResoNix Rope, you can use a proper material to couple the two together and use the rigidity of the crash bar coupled with the viscoelastic and adhesion properties of the ResoNix Rope to provide even more support and structure to the outer skin.
One thing to note, do not rip off the factory material to replace it with ResoNix Rope. Only use ResoNix Rope in conjunction with it. Another thing, more is always better, but we usually provide some gaps to allow for water drainage IF the factory adhesive also has gaps. If it doesn’t, you can go for full coverage. In this case, there was some adhesive gaps so we did the same.
Next up, Blackhole Tiles (As of 1/30/2023 we suggest ResoNix Guardian instead of BH Tiles) for the outer door skin. As mentioned in the guide, what these do is absorb acoustical energy inside of the door. To quote the guide above..
“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. Less energy makes it from the speaker’s rear wave to the outer door skin, and it lowers 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 our Fiber Mat products can work for this, I get nervous about window mechanisms interfering and causing issues. ResoNix Guardian is a safer bet to use here. ResoNix Guardian is a unique product, and there is nothing else like it on the market. We have managed to create a waterproof product that absorbs sound, insulates from heat, and even provides a noise barrier floating inside it. Now, most install this in a checker pattern in their doors, but if you go ahead and do full coverage, you can get some noise blocking and heat insulation out of them as well.”
So yeah, since we have a LOT of energy that will be produced in this specific vehicle, we opted for as much coverage as possible.
Something to note: If you are budgeting to only do partial coverage in your doors, definitely try to get 100% coverage in the area that’s directly behind the speaker and taper out from there.
Another note, since I get asked this more often than I would have guessed.. No, these do not replace a Constrained Layer Damper such as ResoNix Squares. They do two different things and are used in conjunction with each other to reach an end goal. If you had to use one, Constrained Layer Damping is more important.
Once this is done, the outer door skin is complete and we can move on to the inner door skin.
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Moving on to the inner door skin. The most important aspect of treating the inner door skin is to seal the access holes using what many people would call a “block off plate”. Now, after enough time and experience in this field, I can say that while sealing these access holes is the most important part of treating the inner skin, it is highly recommended that you go about it in a way that is easily serviceable. There are many ways to go about sealing these holes. You need something that is solid and rigid. Being able to hold us to the elements is also highly recommended for obvious reasons.
The most ideal way is to use a relatively thick and rigid plastic, usually ABS, and cut out plates that can be secured to the inner skin around the perimeter of the access holes. This is only a good option if the inner door skin is flat and doesn’t have any meaningful curves to it. If it has curves, you are most likely not going to be able to use a thick enough material that will be able to both conform to the shape while also sealing the hole and being rigid and free of resonance. When using a plastic material to block off access holes, You will want to apply CLD to the plastic block off plate, and also provide a gasket for where the plastic meets the metal. I would suggest foam, such as ResoNix CCF7.
The second most ideal way, a way that yields, in my experience, no real audible differences from the first method listed above, is to use ResoNix Barrier as the block-off plate. Our Barrier product, while rigid and strong, is thin and moldable. This is an excellent option to use if your car has a three-dimensional shape to the opening and the block off plate needs to conform to this, or if you do not have the ability to make block off plates out of plastic, as you can cut our Barrier with heavy duty scissors. Using ResoNix Barrier is what we did in this specific install since the inner skin is curved and has a three-dimensional profile to it. In order to secure it, we use riv-nuts secured to the inner skin and machine screws with fender washers to hold it in place.
The third way, and least preferred way, is to just use CLD as your block off plate. While I would say this isn’t a terrible idea for small holes, this is not ideal for anything bigger than say 20 square inches for two reasons. One, it is not what I would consider rigid enough to act as a proper block off plate and cause resonance of its own. Same goes for using MLV for this task. They are just not rigid enough for the job. Another reason, serviceability. Once CLD is stuck on, it is NOT fun to remove. Don’t take that chance.
Once the block off plates are taken care of, you can apply your CLD to the inner door skin. I like to get the block off plates figured out first so I know where to not plate the CLD. You can see the CLD (along with the riv-nuts for the block off plates) installed into the photos above, along with the finished photos below.
Remember, as large of pieces as possible. Little itty-bitty pieces are damn near useless. If the area doesn’t have the ability to accept a piece that is at minimum, and I’m really going minimum here, 8 square inches or larger, do not bother. I typically do not do pieces that are smaller than 20 square inches for the inner door skin. For the door panel and other areas, this is not the case since not every panel is the same and CLD can be used for purposes such as holding two pieces snug together while damping them to prevent their contact rattles. You can see a few examples of this in the reference section above in the CLD installation photos. But if using it just to treat resonance, I tend to not bother with anything that wont take 20 square inches or larger. Once the inner skin is sealed and treated with CLD, you can move on from here and treat any wires or modules that are mounted to the doors using CCF7 to decouple or use as a gasket, and you can use our VW/Audi OEM Non-Woven Tape to wrap any wire bundles to prevent any rattles or buzzes from them. Once this is done, it is time to move on to the door panels.
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Alright, now on to the most complex part of doing a door deadening installation, the door panel itself. This is the part of the doors I get the most questions about, and honestly, I cannot reliably answer most of them since EVERY door panel is different, and every door panel will require slightly different approaches to its spot treatment. I do my best to cover this in the other pictures below that are not of this Lamborghini and hope that is enough to help everyone out there. That said, our goals for the door panel include killing resonance, reducing panel on panel vibration (this includes both the door panel to inner door skin, and the overlapping layers of the door panel vibrating against itself), as well as acoustic absorption to reduce the amount of energy that makes it through the door panel cavities.
Lets start with step one, which would be all of the little spot treatment. Remember, every car is different and you will have to use the information here combined with your own common sense to figure out what parts of this door panel are going to vibrate against themselves and cause audible buzzes.
From our previous door guide..
“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 our VW/Audi OEM Non-Woven 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 CLD Squares on the large, flat surfaces of the panel, we also used the Non-Woven OEM Tape, ResoNix CCF7, and ResoNix 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.”
I prefer to treat this first, because once you apply the larger pieces of CLD Squares, you can no longer take it off and work on the small stuff underneath, so make sure all of your spot treatment is done before you start laying down your large pieces of CLD.
Note: why we used old pictures and other door panels for this is because the Lamborghini door panels are built very well from the factory and didn’t require much spot treatment. This 2014 Mazda CX5, different story.
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Remember, do not waste your time with super tiny pieces unless it is used to prevent two things from vibrating against each other.
Once we are done with spot treatment, we can move on to resonance control of the door. This is when we install the larger pieces of CLD Squares. Remember, focus on the large, flat areas and use as big of pieces as you can to get the most constraining of the panel possible. Again, every car is different and will require a different approach. But my advice comes back down to focusing on the area around the speaker, and any large flat areas. Focusing on the area around the speaker should be obvious as to why, but in case it is not obvious, it is because this is the source of the energy in the door and its always best to stop it or reduce it as soon as possible, same reason why I suggest full coverage behind the speaker with ResoNix Guardian.
Once that is done, it is time for Fiber Mat 25 or 45 (which one depends on how much room there is). One thing to note about Fiber Mat 25 and 45 is that you do not want to stuff it to the point where it is compressed. It absorbs best in its resting state, but there are areas of the door where it will need to be compressed, and is overall better serving the end result by doing so, as it is also acting as a decoupler for the door panel. As you can see, we didn’t just put one sheet of Fiber Mat over the door panel and call it done. While that is fine to do, when trying to get the most out of your install, it is best to fill all cavities and voids to get the most absorption possible (remember, fill, but do not stuff to the point where you really have to compress the material too much).
Thankfully, our Fiber Mat has an automotive grade peel and stick adhesive to make the installation easy.
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Once that is done, it is almost time to install the door panel. But first, the very last thing you want to do is seal the gap between the speaker and the door panel. ResoNix Strips are perfect for this. This couples the speaker to the door panel and prevents energy from getting into the area between the door panel and the inner door skin, and helps funnel that energy out of the door panel grille. Remember, you want your midbass speaker mounted close enough to the panel that the 1” thick strip will contact the door panel and provide a seal. If it doesn’t contact the door panel all the way around and seal the speaker, it isn’t really doing anything beneficial. In the first picture, you can see another Huracan that we did. Here, we used a baffle/speaker setup that allowed the ResoNix Strip to be placed onto the door panel itself. Sometimes its better to do it on the door panel, like in that particular case, but usually it is better to install it on the speaker, as seen in the second photo. We fabricate a ring that acts as a washer over the flange of the speaker and install the ResoNix Strip onto that, but that is a lot of extra work for those without the tools to make that quick and easy, so you can do it how you see fit. You can see the second photo of another Huracan that we did where we installed the ResoNix Strip onto the fabricated ring that secures the midbass driver down.
Once that is done, install your door panel correctly and enjoy your silent doors.
Wheel Well Sound Proofing
Added 11/13/2022
Supplies needed
. ResoNix CLD Squares
. ResoNix Fiber Mat 25 and/or 45 (depends on how much space you have and what you can fit)
. Optional and preferred: ResoNix Barrier
Note: ResoNix Fiber Mat is perfectly fine in this location and exposed to the elements. It will not hold water, it will not degrade, it will not grow mold or mildew. This is the very same material that is used in these locations by car manufacturers such as Mercedes.
Wheel wells are one of the most offensive sources of noise in a vehicle. Below is what I did in my own vehicle, a 2019 Volvo S60 R Design. Please note, that the exact approach will be different on every vehicle. This can be due to how much room there is to work with, moving parts, or more. Please, use your best judgement. We have
First up is obviously removing the wheels and wheel liners. During this process you are going to want to evaluate what you can fit, what parts of the body of the vehicle are going to need the most attention, etc. Remember, resonant panels will need it most. Its crude, but a simple knock with your knuckle will let you know what parts of the body need treatment. More on this part below, but on this particular vehicle, the body was relatively non-resonant, and the wiper fluid reservoir blocked a lot of access on the drivers side. Due to this being planned as a few hour-long project and was recovering from a medical procedure, I did not decide to remove the reservoir and instead worked around it. While getting a closer look, I also noticed that I had gained access to the back side of the fender and the airspace behind it. I decided to also apply ResoNix CLD Squares to the back of the fender and apply ResoNix Fiber Mat 45 on top to fill the airspace.
Next up, treating the wheel liners with ResoNix CLD Squares. Before you do anything, you should clean the liner with isopropyl alcohol thoroughly. In some cases, it is even recommended to sand the plastic with a low-grit paper to encourage the best adhesion possible. In my case, I didn’t feel it was necessary considering how well the ResoNix CLD Squares adhere to pretty much any material and surface you will find in a car.
Once you have the panel prepped, apply full coverage of ResoNix CLD Squares. In the case of doing wheel liners, I wouldn’t recommend less since the liners are mostly decoupled from the body of the vehicle, so not only do you get resonance control of the plastic liners, but it also acts as a noise barrier. Remember, treat all panels in the largest pieces of CLD Squares possible.
Note: We will only be treating the back side of the liner. Leave the exposed side untouched.
Optional: if you have the time and budget, I would also suggest a full coverage layer of ResoNix Barrier over the layer of CLD Squares for further noise blocking. In my case, I did not add this as I was restricted on time, and was recovering from a medical procedure and couldn’t afford the even heavier lifting that Barrier would impose.
Once the liner is fully treated with ResoNix CLD Squares, we can start our full coverage application of Fiber Mat 45. Remember, when it comes to absorption, more is merrier. Full coverage is highly recommended, and if you can fit, the more layers the better. Be sure to take note of suspension components, wiring, or any other parts of the car that may need to be worked around.
Once the wheel liner is covered, that’s it. You’re nearly done and its almost time to re-install it. Before re-installing it, you should explore treating the body of the car with ResoNix CLD Squares. Once that is done, re-install the liner and enjoy 🙂