In a recent blog post I talked a little bit about my noise control strategy. I described the first stage, which is to control panel resonance on the door panels. Most guys go nuts on this step and then stop there. The next step is to add a limp mass barrier, and decouple it from the body structure.
The purpose of the limp mass barrier is to block sound waves, by transforming the vibration into a tiny amount of heat within a membrane (in this case a heavy vinyl sheet known as mass loaded vinyl or MLV). At 1 lb/sq ft, this vinyl is quite heavy, but that is what's needed to block out noise. MLV is often used in building soundproof rooms, and is hung behind the drywall.
How does it work? Well imagine that in the image below, the soccer ball is a sound wave, and the face is a limp mass barrier. The wave hits the barrier, and causes some displacement in the barrier, but the energy of the wave stops at the barrier. The barrier must be decoupled from the body structure so that it remains a limp barrier, if not decoupled, the vibration conducts through the barrier instead of being absorbed. In a car, this decoupling is achieved by adding a layer of foam to the underside of the MLV. So all you have to do is put this mass and decoupler between the noise source and your ear. In a car that looks like MLV and foam under all the carpet, on the firewall, behind the rear seat and on the package tray, and in the door panels if you can fit it.
And so I got to work adding gobs of weight to my car. "Why Jesse, why??" the enthusiast asks. Well here's the thing: this is not a race car, and I'm becoming an old man. I don't need to hear the hum of tire noise, or the drone of exhaust as I cruise down the street. The added performance from a car that is 60 lbs lighter would not outweigh (no pun intended) the benefit of an acoustically pleasant driving experience. Like I said, I'm getting old.
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Melting brain cells, aka gluing stuff together
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Pattern making with chalk
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Footwell MLV installed
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Working on the transmission tunnel barrier
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aaand the other half
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Front barrier complete
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The front barrier proved to be a little challenging, due to my modified transmission tunnel. For reasons I won't attempt to explain here, I needed to join my footwell carpet to the MLV that went over the tunnel. This created a bit of a challenge in joining the varied materials,
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Carpet with mass backing
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The carpet has a mass backing, though it is dissimilar to the MLV and
the vinyl cement which works so well on the MLV bonded to the carpet like scotch tape to a greasy teenager's forehead. That is to say, it was crap. So I proceeded to run tests on samples with all the adhesives I had on hand. Luckily I found that weatherstrip adhesive provided an acceptable bond between the two materials.
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Testing attachment strength
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With the sound deadening barrier covering the transmission tunnel, I was able to apply carpet to the top of the barrier and complete the carpeting in the car.
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Masking to prep for adhesive
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adhesive on the back side of the carpet
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Installed. seams are visible, but it's good enough for me
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For the first time since I've owned this car (which is seven years) and likely the twenty years prior to that, the floor is completely covered in carpet. It feels pretty good, and looks reasonably decent. That's all for today, but there will be more to come soon!
