Long story short, it worked but the results showed otherwise. Worth it if you like to work on cars, not worth it if you're expecting a Lexus LS.
When I was car shopping, I wanted a firm and responsive ride, it had to be manual, and prefer 6 cylinders to a turbo 4. I test drove the Genesis G70 sport manual, but ended up with a used M235i. The G70 is quieter, comes with the LSD, and has better steering feel, but the M235i was better in power, shifter/clutch feel, value (used car), and size. Unfortunately, I found it to have a lot more road/tire noise than I found comfortable. My car has 30k miles and 2014 dated Supersports that were giving a lot of tire noise roar on the highway that I found comfortable, and it was causing a ringing resonance in my ear that was giving me a headache. Tires are not cupped or flat spotted and I just did an alignment.
I previously added sound dampening material to the front doors and front wheel wells of my last DD, a VW Jetta, with great success. It wore 4 year old touring class all season Michelins Primacy. The weird thing is that although the soundmeter app on my phone (uncalibrated) showed slightly larger dB on the Jetta, it felt a lot quieter due to lack of tire roar and that low frequency resonance.
The M235i is my new DD and I have sensitive ears, so I wanted a quiet ride. I was inspired by Crutchfield's youtube video which showed about 2 dB reduction in cabin noise on their Ford truck per segment of roof, doors, floor, etc., for a total reduction of 9 dB, which is a LOT,
https://www.crutchfield.com/S-urlo4b...ound-pt-4.html , so I decided to go all out on this one. I enjoy working on cars and had a few days off.
Before you reply "You should have bought a Lexus", refer to my shopping criteria. I used to DD a Porsche Cayman and while it was definitely the best car I've ever had in terms of driving experience, it wasn't as livable as a larger car, the stereo sucked, it was loud, worried about parking it, often scraped the front bumper on inclines, poor fuel economy, really expensive tires, and it still had a lot more depreciation left than I was willing to eat, etc.
I had identified most of the noise as tire roar, so I made sure to cover the rear wheel well area inside, and the backside of the front wheel well liners. While you can buy quieter tire models, and the age of the 4 year old tires was also a contributing factor, I wanted to save that for a later step and this stuff is my hobby.
So over 3 days, I removed the 2 front door cards, 2 rear seat side panels, headliner, seats, front carpeting, and front wheel wells. I used Nocio 80 mil (0.2 cm) thickness self-adhesive butyl mats, with Nocio green closed cell foam mats, 170 mil (0.4 cm thick). I ran out of the Nocio, so I doubled up on some old ebay soundproofing butyl mat I had. To do all this, I used roughly 50 sq. feet of butyl mat and 75 sq ft of foam, and spend roughly $130 + the leftover mat I had from the previous project (maybe worth $30?), and $30 for the factory door gap foam (the stuff on the 4 cyl and diesels, but not the M235i)
https://www.2addicts.com/forums/show....php?t=1200964. You need a set of plastic trim tools to work on the car, they can be bought from harbor freight for roughly $5.
Some tips: clean everything with rubbing alcohol beforehand to make sure the self adhesive sticks. If adding soundproofing to the wheel wells, wash them first, then wipe down with rubbing alcohol. For obvious reasons, don't apply it to the inner side where it'll get blasted and reduce wheel clearance. I also didn't apply it to the chassis, instead applying it to the backside of the liners. You have to leave room to reattach the panels. Additional material turned them into an effective sound barrier to help reflect sound away. BMW made the rear section foam instead of hard plastic like the front - I just made it an even better barrier. The roof already had carpeting on it and I didn't want to remove it, so I added foam around it and to the headliner. In retrospect, I would have pulled the carpeting since it was falling off in the corners already, cleaned thoroughly since the mat will be upside down, added some butyl mat to the roof, then used new spray adhesive to reattach the carpeting.
My foam also wouldn't adhere to the backside of the headliner since it's a fibrous material, so I used some 3M 77 spray to stick it on. I also didn't feel like breaking the shifter so I worked around the center console instead of removing it completely. This led to some gaps under the console, as I only did the sides around the transmission tunnel instead of also the top. I also added the OEM foam seal between the fender and chassis, you can find details on this mod elsewhere here.
Regarding disassembly, I had trouble with was the 3 plastic expanding rivets on the outer lip of the wheel well liner at the fender. Push the middle part into the fender and the rivets will release. Then find the center plugs behind it for reuse. Mine were dirty and it was getting dark so I tried to pry them out and ended up replacing them. I also couldn't figure out how to remove the upper console on the headliner since it was different from the manual. Mine had 2 metal spring clips near the rear - use a screwdriver to simultaneously push both towards the midline of the car, then the rear will drop down.
In general, the butyl rubber mat was not effective at blocking sound. It's for mass loading and reducing vibration of the material. If you only use butyl mat the results are going to be very disappointing. It also doesn't have to have 100% coverage, only enough to change the resonance of the material. Dynamat recommends 30% coverage but a little more is better than too little. The car has tiny square from the factory and they're really inadequate, but work. The closed cell foam blocks and absorbs noise, but it has to have 100% coverage, or as much as you can. I found much better results from the foam vs. the butyl mat. Tire/road noise is mostly low frequency, so that's what you want to target with mass loading and blocking the path of noise.
I used my soundmeter app (uncalibrated) on my phone to measure before-after results, drove with climate control/radio off, and used the same ~1 mile stretch of highway, always in dry pavement. The phone was held above the shifter. There was often up to 2 dB variation in samples. This could be from ambient temps affecting tire noise, a loud car next to me, windy day, etc.
60 mph (n=5), mean dB of 68.7
70 mph (n-6) dB=71.6
70 mph (reverse trip) (n=4) dB=70.5
after adding sound dampening material
60 mph (n=2) 69 dB
70 mph (n=1) 70.4 dB
Even though I only had a few samples, it was enough to show me that I wasn't going to see the 9 dB that Crutchfield got. I suspect their Ford F150 had much less soundproofing to begin with, and they used a lot more material. For example, their test vehicle only had thin plastic as a water barrier behind the door card. The M235i has at least some foam. I also don't know exactly what speed they had this 9 dB reduction at. How many trials did they run? Was it over the same stretch of road, under similar conditions? In driving under different roads, I found 5 dB difference in that alone. Their video showed mostly low speeds and maybe they were driving over rough road to accentuate the difference? I also ran out of nocio mat, which I found very sticky and heavy (I found it to be a good product), so I used my older ebay mat, which wasn't as sticky and dense, on the wheel wells and rear panels, but I doubled it up to try to make up for it.
However, the car feels much quieter to me, and since the resonance of the tires on the highway was greatly reduced, and it's much quieter at 40 mph, I partially accomplished my goals. It just wasn't as much as I'd hoped, especially for the amount of work put in. I think it's because the car already has decent sound dampening. If you were doing this to a Corvette or Subaru, the lack of sound insulation on those cars would be more pronounced. I felt that adding a small amount under the thick foam and carpeting of the floor didn't do much, but the rear-sides and doors did, since those areas were the most uninsulated. I just can't back up my observations with data since I took my measurements at highway speeds which is where the tire noise and resonance noise was most offensive. I suspect if I took measurements at lower speeds I may have found some evidence. Around town, it's pretty quiet with very little road noise, and I feel the most noise reduction was up to 45 mph, not the low frequency highway roar that I wanted to reduce. Also, the dB meter app doesn't show what frequencies are reduced - low frequencies are good at sneaking by gaps, so I suspect the offending noises were reduced less than other noise. The noise character definitely changed.
What I wouldn't do again:
The BMW has thick foam under the floor carpeting which is way more effective than the 0.4 cm of foam I added, so I can't say if doing the floor made much, if any difference. Pulling the seats is easy, but the center console is more difficult. I can't tell if adding foam to the headliner and roof made much difference and doing headliners is best for reducing wind noise, not the road-tire noise that I found offensive. I just had leftover foam and used it up, but also hoped that perhaps the roof was reflecting noise back down into the cabin, and that this would help, especially since the roof has such little insulation. I can't tell if it did anything. The area under the rear seats is quieter, but not by much. The rear seat foam does a good job of absorbing noise.
What I would do again: The doors made a very significant difference. I added some mat and foam to the outer skin, and added complete foam coverage to the door panel. The stock HK system sounded SO MUCH better(my center channel is unplugged). The noise from the rear cabin is much less, which I attribute to doing the rear seat sides. They're easy to access and I added mat+foam to the outer skin, the wheel well area, and the inner metal skin. There also isn't much soundproofing in that area and I definitely noticed it. This is versus the area under the rear seats, which is covered by thick foam seats.
Attached are pics showing before and afters. Some were the 1/2 way point: everything except the headliner got both butyl mat+foam. The backside of the wheel well liner had spaces where could fit mat+foam without interfering with fitment, which is why it looks so patchy.
Future plans: once the supersports are worn I'll get quieter tires. New tires are usually quieter due to fresher rubber alone, but the Michelin 4S is supposed to be better than the supersports in every way. I'm not going to use all-seasons, not worth the performance drop for me, especially since living in the northeast, need to use a separate set of winter tires. Thanks for reading!