How To: Sound Deadening Car Doors The Right Way

ResoNix Sound Solutions What Is Sound Deadening

How To: Sound Deadening Car Doors The Right Way

Sound Deadening Car Doors: Here at ResoNix Sound Solutions, it’s no secret that end-result performance is our main priority. It’s also no secret that there is a lot of misinformation, poor advice, and marketing disguised as guidance that plagues the automotive sound treatment groups, threads, and discussions. The point of this article is to show you how we here at ResoNix sound deaden car doors for the best end result possible. This post is taken from the “Installing It: Doors” section of our Reference Information & Guide page. The installation is done in a Lamborghini Huracan Evo, but applies to pretty much any vehicle.

 

NOTE: This guide is for SOUND DEADENING your car door, not sound proofing. There is a difference. Sound deadening your door, which is what we are covering in this article, is the approach you would take when you want to treat the part of the vehicle, in this case the doors, for audio performance improvements only. Sound proofing is the approach you take when you want to reduce outside noise from entering the vehicle. The approach is similar, but there are some small differences. For the doors, the only difference would be a full application of ResoNix Barrier added to the inner door skin after CLD application.  

Update: ResoNix Video Playlist Of Sound Deadening Car Doors

Installing it: Sound Deadening Car Doors

New Version. Written 12/17/2022

       Hey everyone, Nick here. I finally have an updated guide for you on how to sound deaden car doors. This one being in a Lamborghini Huracan that we did. This new guide features almost everything you need to know and need to see to sound deaden 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 on how to sound deaden car doors 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 on How To Sound Deaden Car Doors

     100%, or near 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.

       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.

       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 On Sound Deadening Car Doors

       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.

Removing The Access Panel & Door Speaker

       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.

ResoNix Sound Deadening Treatment On The Outer Door Skin

       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. 

        Let’s 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..

ResoNix Butyl Rope Door Installation

       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.

Absorbing The Midbass Drivers Rear Wave With ResoNix Guardian 

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.

Using ResoNix Sound Deadening To Treat The Inner Door Skin

       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.

Using ResoNix Barrier To Create Door Block-Off Plates To Cover Access Holes

       The most common 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.

       Another but lesser-known ideal way, a way that yields, in my experience, the same end result 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.

Using ResoNix Sound Deadener To Treat The Door Panel

       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.

       Remember, do not waste your time with super tiny pieces unless it is used to prevent two things from vibrating against each other.

Using ResoNix CLD Squares To Reduce The Resonance Of Your Door Panel

       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.

Using ResoNix Fiber Mat To Absorb Sound & Decouple Your Door Panel

       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.

Using ResoNix CCF Strips To Direct Sound Energy Out Of The Door And Into The Listening Space

       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.

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