Block Airborne Sound
You'd be forgiven for thinking that vibration damper was the most important treatment. It certainly gets most of the attention. Doesn't matter. It's the barrier layer that makes the most difference. I use mass loaded vinyl (MLV) for this. MLV is the answer to the question: "How inexpensively can we create a substitute for lead sheeting that will accomplish the same thing"?
Mass loaded vinyl is dense and limp - the ideal characteristics for a barrier. A material's resonant frequency is determined by its mass, stiffness and geometry. Low mass/high stiffness mean high resonant frequency. High mass and low stiffness mean a low resonant frequency. MLV and lead sheeting have resonant frequencies below the the audible range. Makes it an excellent barrier.
Build the Bubble
Single most important concept and the most important tool we have. Barriers work best when they form an unbroken shield between the noise sources and your ears. CLD Tiles are designed to work best with just 25% coverage. That isn't true for a barrier. For best results, you will want to completely line the floor, doors, quarter panels and trunk or cargo area for a hatchback, station wagon or van.
That's everything below the glass line. The glass will always be the weak link. That's why there's very little point in installing a barrier on the roof. It would only be effective for sound coming from directly overhead and there isn't much of that. Fortunately, most of the noise hitting a vehicle is coming at it from below.
Probably be better to think of it as building the bottom 2/3 of a bubble. To the extent possible, you want a layer of mass loaded vinyl all over that bottom 2/3.
Now that you've got the image of the protective bubble in your mind, you need to decouple it from the vehicle. That usually means putting a layer of 1/8" closed cell foam (CCF) between the MLV and the vehicle, trim panel or other hard surface. The important thing is to separate the MLV layer from any hard surfaces that may be vibrating. Direct contact would make the MLV part of the problem. You also get enhanced high frequency attenuation by presenting a more complicated sequence of boundaries to sound trying to penetrate the dissimilar layers.
Wouldn't it be Easier to use an MLV/CCF Composite?
At first glance it seems like a no brainer. It has to be easier to install an all in one product, right?
It is orders of magnitude more difficult to install an MLV/CCF composite in a vehicle, anywhere but the bottoms of the floor pans. Here's the problem:
You will need to combine pieces of MLV to cover compound curves. MLV is very flexible - in one direction at a time. There is no way to force it to conform to a compound curve.
It's also critical that there be as few gaps in the MLV layer as possible. There will need to be openings to allow mechanical elements and cables to pass from one side to the other. You want them to be as small as possible. Where there are seams in the MLV, you want there to be no gap in the 1 lb/ft² coverage.
There will be places where it's simply not possible to fit CCF with the MLV.
Finally, you will need to cut your materials precisely. MLV and CCF present different cutting issues. It is almost impossible to be precise when cutting both layers together.
Not All MLV is the Same!
Any 1 lb/ft² flexible material is going to have similar performance as a barrier. Tensile strength and flexibility are worth considering but odor is the biggest concern. I've been trying to figure out why some MLV is very inconsistent between batches and why the only MLV that is consistent costs me more by the ton than most online sellers charge per square foot. I guess I've been doing this for long enough and am selling enough MLV to be considered "on the inside" by my manufacturer. Finally got an explanation that makes sense.
Most MLV is called "regrind". The vinyl component is made from recycled material. That may be a good thing for the environment but it makes it very difficult to control the exact characteristics of the finished MLV. A persistent odor, less flexibility and lower tensile strength apparently aren't deal breakers when the MLV is sealed inside the walls of a building. They are for me in a vehicle. The MLV I sell is made from virgin materials for consistency and quality.
At 1 lb/ft² it is heavy to ship. I'm on the East Cost. It currently costs more than $100 dollars to ship a 135 ft² roll to California. It can be worth looking for a local source. Just be sure you can handle and smell the material before you buy it. Three customers have removed the MLV they bought somewhere else from their vehicles because of the smell.
What About Hardware Store MLV?
Hardware Store MLV isn't anything like what we've traditionally called MLV. Calling it MLV is confusing but I do see it referred to that way on forums quite frequently. The sellers don't call it that. It's made from EVA (Ethylene-vinyl acetate), so the word is in there but its characteristics are completely different.
The first concern is that it's very stiff. With that comes a higher resonant frequency - we really want a limp barrier to keep its resonant frequency below the audible range since materials are acoustically transparent at their resonant frequency. This stiffness means it is only usable on completely flat surfaces.
The second problem for our application is that EVA is not compatible with any reasonable adhesives that can be used by DIYers. There's no good way to seal seams and combine pieces. That's a problem because of the irregular contours of vehicle interiors.
Finally, EVA is hard to cut with any precision at all. It's meant to be nailed to walls or under floors and cut with carpentry tools, but mostly just placed end to end with little cutting needed.
|mass/area||1 lb/ft² (4.88 kg/m²)|
|Nominal Thickness||.107 inch (2.7mm)|
|Tensile Strength (PSI)||762|
Sound Transmission Loss for 1 lb/ft² Mass Loaded Vinyl
|Sound Transmission Loss||15||19||21||28||33||37|