Sound proofing to reduce road noise

Sherri Zann · 02-25-08, 01:07

I'm looking to see if Autopians can clarify the issue of how to efficiently reduce noise in cars. This is one of the best sites on the web for intelligent inquiry about cars, and this is a topic with a lot of half-baked posts on other sites.

I've been doing a lot of research on noise reduction in cars. On many sites, the Dynamat or other heavy matting is put on every square inch of interior door skins and floor panels. Maybe a thin bit of closed cell foam is then put on top of this.

But when I look at scientific research on auto noise reduction, it looks like recent computer models are showing how to REDUCE the amount of metal damping to optimize weight/noise. In Europe, they discovered that they could get better sound damping and save both bulk and weight by using thin layers of dense and less dense felt to absorb sound. Here's a press release about an interesting recent study by the EU: Smart sound insulation in cars leads to lower environmental emissions

I just used old yoga matting in the A-pillars of my civic to reduce squeaks and deaden the sound.

Anybody here have ideas about efficient and effective noise reduction in cars?

Danase · 02-25-08, 01:37

Also look at what tires you are running. I tossed my stock Bridgestones on my old Impreza for a set of Avon Tires and the car got sooooooooo much quieter.

CleanGSR · 02-25-08, 01:54

Good topic. I'd also like to know if anybody has tried those spray on sound deadners with any success in noise reduction. I wouldn't mind using Dynamat, but it's expensive and really heavy. I really don't want to add a bunch of weight to my car.

the other pc · 02-25-08, 04:38

Hey, don’t get me started!

I mean that literally. Please, don’t get me started. I don’t have the time to dive into another new hobby/field of endeavor/relentless obsession.

I’ve long been fascinated by sound and vibration but I’ve never really gotten past the basics. It’s a hugely complicated corner of science and engineering.

The reason you’ve seen a lot of half-baked posts on the subject is that it’s so specialized. Most of the people you see posting about it are amateurs, enthusiasts and Joe-Blow-off-the-street guys who are playing and experimenting without much actual knowledge.

There aren’t all that many people out there with real expertise and they aren’t hanging around in online forums posting everything they’re doing. You’re mostly going to find their work published as academic or industrial research.

Which is not to say that hobbyists can’t be successful, many are. It’s just that you can’t easily differentiate their “dude, this sounds much better” posts from the next guys’ because there’s no objective basis of comparison.

Just as a taste of what goes on in the field:

Automotive DesignLine: Design tools quiet automotive noise.

Automotive DesignLine: NVH analysis methods quiet auto cabin noise.

samples of SAE topics on acoustics and NVH.

PC.

Sherri Zann · 02-26-08, 08:25

"Dude, this sounds much better." Lol. Yep. That about sums it up. But, that's what I want! I want it to sound much better!

Bob, your post caused me to go over to Tirerack.com, and since my car is a Civic, to some of the Honda forums where there were threads about quieter tires. Lots of posters agree that some tires are much quieter than others, and that some of the quietest can also be among the best handling. Tirerack.com didn't offer any Avons as fitting my car. But tires that were repeatedly mentioned on forums included Michelin Primacy MXV4 (new, $100 ea.), Bridgestone Teranza, and Goodyear Assurance Comfort (80 ea). I've still got enough tread on my factory Firestones that I don't want to chuck them quite yet.

Part of the noise in my car is from interior squeaks and rattles. There was a dash rattle that the dealership could never fix, though they heard it. Out of desperation, I pulled off the trim from the A pillars this past weekend, and found that the front edge of the plastic dash on the passenger side was poorly supported and bouncing. This was the rattle-maker. Some dense felt between the corner of the dash and the windshield wedged the dash in place and eliminated the rattle.

Also, I found that the hollow A pillars seemed to conduct sound into the cabin. I am co-owner of a yoga studio, so we have old yoga mats around. They are a very dense PVC foam reinforced with thread. I used the A pillar trim as a template to cut yoga mat to fit between the A pillar and plastic trim. This essentially made a sort of full-face gasket. I had to trim away about a nickel size hole in the mat around the two bayonet trim fasteners in order to get the plastic trim to fit back and sit tight. Also, everywhere that the pillar's plastic trim touched a hard surface, like the dash, I adhered some thin felt to at least one of the surfaces. I had some old self-adhesive felt, but the adhesive had dried out, so I used rubber cement. Nice thing about the rubber cement is that when it is dry, you can remove any excess from the dash just by rubbing.

Two very good results of what I did. The yoga mat seems to both block and absorb reverberation from the A pillars. You can rap them and it is "dead." In addition, the felt between the A pillar trim and dash has quieted the creaking and buzzing a lot. It seems tight and more like a solid unit.

I'm going to read the articles you posted, PC. Thanks.

Sherri Zann · 02-26-08, 08:46

"A visual inspection of the trim panel showed that a spacer acted like a bridge, allowing vibration transmission from the door module to the trim panel. In this case, structural vibrations were transmitted from the door model to the trim panel and were further transformed into acoustic energy that radiated out as sound. " Design tools quiet automotive noise: Part 2 - an electric motor application | Automotive DesignLine

PC, this link from the original article is fascinating, and indicates that if you stop the transfer of vibration from the metal panels to the interior trim, you can really quiet sound quite a bit. This is very different from just plastering every interior surface with heavy mats like Dynamat or Second Skin.

"The results show that there was very little sound coming from the motor directly. The sound was emitted from the door module. In other words, the motor was the source of vibration energy, but the source of sound was the structural vibrations of a door module. Therefore, any attempt to modify the design of the motor will be a waste of time and effort. The focus should be on isolation of the vibration transmission from the motor to the door model.

The results demonstrate that the [electric window] motor is the source of energy in the system, but not a source of noise. Most of the noise is produced by structural vibrations of the door module. The motor then makes a door module vibrate, which acts like a speaker."

Pretty interesting and practical stuff.

BlueZero · 02-26-08, 09:05

Tires do seem to make a huge difference with road noise. Now if only I could stop my passenger seat from squeaking. It's driving me crazy, crazy I tell you. lol

I've always wondered about using Great Stuff in areas like the A pillars. Make sure to try the window and door version since it doesn't expand as much as the regular. Sort of a crazy idea but maybe it would work.

Sherri Zann · 02-26-08, 12:17

Yeah, there is some outfit in Manila (?!) called "AutoFoam" that shoots expanding polyurethane foam (from Germany, probably Wurth Chemical as source has been the speculation) into the car pillars and sills. There is a bulletin board where a number of people either had AutoFoam do this, or they took Great Stuff and did a DIY job.

Great Stuff is polyurethane-based, and yields a closed-cell foam. Make sure you don't use a substitute which is latex-based, because this creates an open-celled foam that can absorb water.

Also, here's a warning from the door and window version of Great Stuff (GS): "Reacts slowly with water, releasing carbon dioxide, which can cause pressure buildup and rupture of closed containers. Elevated temperatures accelerate this process." My concern would be if you really sealed a hollow section, having the car heat up in the sun and potentially push out a panel. Of course, that's what you are trying to avoid by using the w/d version that has minimal expansion, compared to the gaps and cracks GS.

Also, GS is about 20-30% isocyanate, and you don't want to be breathing the fumes while it is curing.

I was very, very tempted to shoot GS into the A pillars and the sill. I haven't so far because of my concern that I might end up trapping water inside the car structure and helping it rust from the inside. For example, I have a sun roof. And there are all sorts of other ways that water can get inside structures. I wonder if the open pillars are part of drain paths. I may examine the sills and pillars more carefully from below to figure out how this really works. I think the idea of foaming these hollow areas is potentially excellent, both for structural rigidity and noise-reduction. The guys in Manilla were very enthusiastic after doing it!

BlueZero · 02-26-08, 12:38

Interesting! I wonder if it would pay to fill a soda bottle with some and put it out in the sun just to see what happens maybe another bottle with a little water in it.

Sherri Zann · 02-26-08, 12:50

If the cap is on the soda bottle, I think it will burst.

Water in the bottle when the Great Stuff is first shot in will actually accellerate the foam reaction. Once dry, the GS is closed cell and has a skin on it, and shouldn't suck up and hold moisture.

My concern was more that the Great Stuff would simply be a physical barrier in the way of water that got into the car structure, so that it couldn't drain out the bottom.

Brandon1 · 02-26-08, 12:59

You can also use something called "peel and seal" to sound proof doors and floors. It's a aluminum backed tar strip, kinda like sound dampening material. I use it to deaden doors for better bass response in car stereo systems. Do your doors and floors with the stuff, prolly only take 4 rolls, each roll is $13. Available at Lowes.

Sherri Zann · 02-26-08, 01:39

Hey, Brandon! How are you?

How has your peel and seal held up to summer heat? Have you been through a Carolina summer with it?

Take a look at Sound Deadener Showdown
This is an impressive testing of sound deadening mats and how they hold up to heat. After the reading I've done on mats, I wouldn't use an asphalt-type product like Peel n' Seal or Dynamat Original.

I'll probably use some mat on metal panels--but not slathering it on the whole surface--and I'll be using Second Skin's Damplifier or Raammat. With all the labor involved, it just doesn't strike me as that much more money to get the good stuff.

But, that said, if you've had a great experience with the cheap stuff...maybe I could be persuaded!