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My apologies for my long absence and lack of updates. The wildfires in Oregon last month which caused raining ash and choking smoke and then the heavy rains that followed were completely unsuitable environments to work on the car or do testing, so I had to pause for a while.
I did manage to take some more measurements and to install the Mercedes OEM damping materials from the 839 package in the rear floor and transmission tunnel area, but first a very important note about the dB testing:
I initially chose three different stretches of road: very smooth asphalt at 65mph (to best analyze wind noise), moderate asphalt at 65 mph, and concrete at 75 mph. The portion of concrete freeway I chose is now being repaved, so no further testing is useful on that stretch. I'm honestly a little thankful as the lack of convenient exits and relatively heavy traffic made the testing there quite bothersome. I'm also no longer going to be testing on the very smooth asphalt as all modifications except the windows only focus on road noise. Only having to test on one stretch of road makes the measurements worlds easier and makes the data analysis go more quickly as well.
Also due to the time-consuming nature of trying to test in three different areas and only being able to work on the project sporadically, the tests have been very spaced out in time with respect to each other, often with many weeks intervening. It's important to note that I drive about 800 miles per week on average. When my previously posted results showed that the windows reduced noise at some frequencies but increased it at others, I entertained some acoustic theories as to why that could be the case. However, when I recently measured the results after my weeks-previous installation of the absorbing rear wheel-well liners and wheel arch damping (which noticeably quieted the car subjectively), I found that noise increased somewhat at nearly all frequencies, for which there are no plausible theoretical explanations that I can think of with respect to the new modifications. Looking back at patterns in the data, I now believe that the long time (and thus many miles) between measurements has lead them to be irrevocably confounded due to increasing tire noise from tire wear. The future solution to this problem is simple, the baseline before any modification and the measurements after should be conducted as close in time and mileage as possible. This means that all baselines will be relative, but differences can be summed in a mathematically suitable manner. The bad news is that the initial baseline measurements (taken all the way back in July) are essentially meaningless as a point of comparison for the modifications and I don't think that I will ever be able to get a complete idea of the overall improvement for the project.
However, there is also good news. I installed the remainder of the 839-associated Mercedes OEM damping pieces in the rear footwell and transmission tunnel area. This was a little easier than I had anticipated as we managed to do it without entirely removing the rear carpet. We removed the rear seat cushion, lower B-pillar trim, rear inner door sill scuff plates (these broke and need to be replaced), and entire center console. We didn't remove the front seats, but we did have to remove their rail bolts to tilt them forward enough to pull up the rear carpet around the transmission tunnel.
The large aluminum-backed sheets on the outsides are the MB-specified sheets for the 839 package. The smaller ones in the middle are also MB OEM, but leftover since I couldn't access the internal wheel-well area where they're meant to go. So I figured I could use them here instead. I also used leftovers to put 4 instead of 2 on the transmission tunnel and 2 on the front of the rear seat hump facing the passengers' calves. No photos of the ones on the transmission tunnel as the space was far too tight.
Also worth noting is that the carpet is very thick and has thick, dense molded foam underneath it. Overall it is of excellent quality and should have good acoustical properties.
I took pre- and post-damping installation measurements within a few hundred miles. The results are excellent and exceeded my expectations for the relatively small amount of material applied.
There is a broadband noise reduction with an overall A-weighted reduction of 1.3 dB. The reduction exceeds 3 dB around 800-1000 Hz where the ear is quite sensitive. This is quite worthwhile given the low approximately $150 cost including the extra pieces, plus about $50 for the broken sill guards. The installation is laborious and you have to be willing to remove the center console and unbolt the seats, but I can confidently recommend this modification alone for anybody hoping to reduce W212 road noise. Subjectively the car sounds smoother and the road noise is less harsh with a more homogeneous quality. There is somewhat less perception of road surface changes and concrete in particular is less objectionable. The perception of the location of the sound shifts to the outer edges of the rear of the car, probably reflecting the reduced floorpan and transmission tunnel noise towards the center of the car. Overall I am extremely pleased with both the measured and subjective performance from the OEM rear footwell and transmission tunnel damping retrofit.
While the center console was out, I also removed the button strip with the heated seat controls and tightened the screws on the back of it and lubricated the plastic with a Krytox spray. I also lubricated the plastic-on-plastic contact points where it mounts with Krytox grease. This greatly reduced any squeaking/crunching sounds from the area.
The next stage of the project is adding aftermarket damping and 3M Thinnsulate in the DEF tank/spare tire well area and behind the trunk panels as well as adding decoupled MLV behind the folding rear seat and across the trunk floor.
Very interesting! I just got the rear carpet out of my C and will start putting down the 'Siliss" material tomorrow. The floor is all bare metal in my car; only molded foam on the back of the carpet provides any sound deadening.
Also took off the B pillar lower section and will put Siliss in there behind the seat belt. May also shoot expanding foam through the holes to fill in the pillar entirely. There shouldn't be any openings that could admit water into the pillar, so I'm not worried about moisture in there.
Will post some pics tomorrow showing ... it's encouraging to hear you think this is a productive area to work on.
Just in case you want to use Rockwool at that fender area between wheel well and front door, which we once discussed.
Just for info. Its new to me too, since I never actually pay attention to wool insulator moisture issue.
Just in case you want to use Rockwool at that fender area between wheel well and front door, which we once discussed.
Just for info. Its new to me too, since I never actually pay attention to wool insulator moisture issue.
Thank you very much for the tip. I think that, in addition to some CLD in the upper and cabin-facing areas, would be idea for that area if I choose to pursue it. Most of the noise still seems to be coming from behind the rear seats though, so that's still the focus area for the time being.
Through this project I've learned a tremendous amount about how to not measure SPL inside a car. Should I embark on a similar project with another vehicle I think that the measurements and quantification will be far smoother and more useful as I will have worked out so many confounders and solved so many procedural and processing issues during this project. However, while developing this knowledge of problems and their solutions in the quantification is great, the sad part is that I don't think I will ever be able to quantify the beginning-to-end outcome of this project very well since the methods (necessarily) keep changing.
The latest stumbling block in measurements was noticed in patterns of acoustic spectra differences between measurements taken in the early morning and late afternoon, the latter having warmer ambient and road temperatures. I noted substantial, broadband reductions in noise at higher temperatures. I reviewed previous data and even conducted an extra set of measurements between modifications to confirm my suspicions. This academic article backs up my observations by quantifying temperature reduction in tire noise at increasing temperatures of an average -0.107 db/C.
Therefore, in order to truly have an apples-to-apple comparison, the measurements should be done at the same time of day in similar weather conditions and with as little mileage in-between as possible. Practically, this means measuring on the morning of project day and then the next morning, assuming that the weather cooperates. I have been able to apply this methodology to my latest modification, however, which is covered in the next post.
Much of the discussion on this forum and my own empirical listening have pointed to the trunk and rear tirewell area as a source of noise. I think that the folding rear seat assembly is part of the problem here, I wonder if MB NVH engineers primarily tested on cars without the folding option which was later made effectively standard as part of the Premium Package for the US market.
I have little use for the folding seats, so I decided to construct an MLV noise barrier that spans behind the rear seats and as far out to the sides of the car as possible: behind the rear seat side bolsters and under the trim to their outsides. It also tucks under the hinged rear seatbacks.
I lined the outer edges with 3" wide strips of 3M automotive Thinsulate. When I swap the rear seatbacks in my leather interior conversion project I want to remove the MLV piece and line the area of it that is visible from the trunk in black microsuede fabric for an improved appearance in the trunk.
I also damped the metal under the rear seat cushion with constrained layer butyl-rubber and aluminum foil (CLD).
Using my new measurement rules detailed in the previous post, I measured the sonic improvements. It's a small but broadband improvement in noise, with particular improvement in the upper midrange. Subjectively it is larger difference than either the rear floorpan damping or fender liners and makes a significant improvement in comfort. Road surface changes are substantially less intrusive and my partner believes that the car is indeed quieter. At this point, between all the modifications, there is a substantial overall difference in acoustical comfort. I now almost never find myself bothered by noise on the freeway, while it was very common before the project. The noise that remains is of a much more homogenous, "smooth" nature. The harshness part of NVH seems to have improved perhaps even more than the absolute noise levels.
I live in Oregon where all freeways have heavy aggregate for traction in wet conditions. Unfortunately this also translates to very high levels of road noise in most vehicles and my 2015 E250 Bluetec is no exception. The noise is a dull roar at highway speeds, not a tire sizzle. The experience just isn't commensurate with my expectations for an E-Class. I drive about 800 freeway miles per week and the noise becomes quite wearing eventually.
I know that many forum members have had similar issues with the W212 and I'm quite certain that I've read every thread on the topic at least a few times. Several areas of interest have caught my attention:
Rear Tirewell Area
The most common focus is the rear tirewell area. I've read of forum members applying all sorts of damping and/or absorption materials in the tirewell with remarkably mixed results. Some claim that they realized huge improvements while others claim little improvement. I decided to do an experiment. I filled all the empty space around my DEF tank with towels, except for the DEF heater and AUX battery. I then replaced the trunk cover and my WeatherTech trunk liner, put down about 2" of foam I had laying around on top of the liner, pushed pillows and a comforter up against the back of the rear seats, and then put another comforter over the foam. I figured this makeshift hodgepodge should perform within the ballpark of more permanent but much less bulky acoustical treatment. I then performed a test. I chose three short (about 60-120 second) routes and drove them at 65 mph, each twice. I used the DecibelX iPhone app and while the accuracy may not be great, the precision of the phone and the testing method was good with each run's average measuring within 0.2dB or better of its duplicate counterpart. I then remove the absorbing materials and repeated the six runs. I realized about 0.5dB of improvement with all the materials, not nearly as good as I was hoping for.
I'm not sure if its worth it to continue to pursue noise control in the tirewell area. I don't know if having a DEF tank makes the noise control harder or not, as many of the members reporting improvements removed their spare tires to conduct the treatment. I did tap on much of the sheetmetal in this area and it is mostly non-resonant. This makes me think that a successful solution is more likely based on sound BLOCKING, as opposed to stick-on deadening. If I were to pursue this area further I would likely line the area with Second Skin Audio's Luxury Liner Pro, a noise blocking mass loaded vinyl (MLV) with a foam backing to decouple it from the substrate surface. I would then fill empty space with 3M auto Thinnsulate and then also apply the Luxury Liner Pro on the bottom of my WeatherTech liner. I might even fashion a blocker to go behind the fold-down rear seats. However, considering the poor results of my first test I don't know weather this would be worth the ~$200 cost and time to test it. I would welcome any comments or experience on if I should proceed.
One forum member, @S-Prihadi , has replaced his OEM Sachs shock absorbers with Bilstein B4 and claims an improvement in noise overall. Although I don't know if it is the same kind of high-speed tire roar that is bothering me so much.
Any experiences with suspension and road noise in the W212 would be very useful.
Floorpan Treatment
One member reports having Second Skin products professionally applied to the floor. I suspect this could have a significant impact, albeit at significant cost and weight and a tremendous amount of time to disassemble and reassemble the interior. I believe the seats and center console must be removed to remove the floor carpets. I cannot find documentation for this procedure. Documentation about removing the carpet and/or experiences adding acoustical products under the floor carpet would be great.
Laminated Safety Glass with Acoustic Film (839) was an available option from the factory. I have spoken with my local dealer and might be able to order this glass from Germany. Between the glass, retinting, and installation the cost is significant, if not prohibitive. I'm certain this does reduce noise transmission through the window, but the question is what kind of noise is transmitted through the windows? My guess is more wind and traffic noise than tire roar, so this is likely something I would do after making a decent dent in the tire roar noise by addressing other issues.
Door Seals
I know the door seals eventually wear out and will allow sound into the cabin. However, mine are fine on visual inspection and I only have 32k miles. Also, there are no easily identifiable noise hot-spots that would indicate an area of poor sealing. Instead the sound just seems to be coming from everywhere.
Any comments or experience with reducing W212 road noise would be greatly appreciated. Obviously there are a lot of possible strategies as detailed above. I'm currently a bit discouraged at the failure of my initial experiment and trying to decide what to try next.
a comment on the sound dampening methods: I investigated this back studying speakers. One needs mass to stop all sound frequencies. Foam (blankets, pillows, fiberglass) stops high frequency sound. Don’t be fooled by the second skin people. Get straight MLV. Mass loaded vinyl. 1lb/sqft from eBay. A single layer of that on the floor and doors, and a double on the wheel wells will make a difference. Or just get an S class
hmmm. We have a 2016 'luxury' wagon, and certainly notice the tire noise from the Continentals mostly from the back. today I drove it 80 miles or so with the back seats folded and not much in back and it was even more obvious.
I wonder if any of this sound proofing could be appled to the wagon? thinking dynamat kind of stuff under the floor/spare in back... the wagon has a 3rd row of 2 kidsized seats in back under the floor.
hmmm. We have a 2016 'luxury' wagon, and certainly notice the tire noise from the Continentals mostly from the back. today I drove it 80 miles or so with the back seats folded and not much in back and it was even more obvious.
I wonder if any of this sound proofing could be appled to the wagon? thinking dynamat kind of stuff under the floor/spare in back... the wagon has a 3rd row of 2 kidsized seats in back under the floor.
i added dynamat to the inside of the tailgate of mine and it made a notable difference
I did the rear of my w212 with two products.
did the spare wheel area, wheel arches and under rear seats. The results are very very good. Use these two products. Dodo Matt hex and CTK 8mm sound absorber. Quite amazed how much the sound has dropped. I also have a Mercedes van and covered the bulkhead it that again can’t really hear the back of the van anymore. Will do the doors in that next. Not sure really need to do the car doors. But would recommend these products. I have covered as much as could in both. Took a couple of hours for the car and about the same for the van. Money well spent.
10-03-2020, 12:56 AM
