HOW does dynamat work?

[jonathanengr1 10+ year member]

I have searched and searched for an answer to this question and can't seem to find a comprehensive answer. There are numerous posts about "DOES it work?", but nothing about the theory behind how it works.

Thus-far, these are the two theories that seem to be eluded to:

1) By adding weight to the panel, it lowers the resonance frequency of the panel thus eliminating vibrations and "rattle"

2) It "stiffens" the panel thus reducing vibration.

There is also talk about it having soundproofing qualities, which doesn't appeal to me. Here's my application:

I have a pontoon boat with a pair of 10" DD subs enclosed in an area (area is roughly 2 feet x 2 feet x 3 feet) where three sides of the area are aluminum panels. I finally got the subs performing *incredibly* well (thanks to those who helped me out in the sub forum!), but when it hits low it's almost comical how much the panels kick in and out (looks like the cone of the sub), and at lower frequencies (lower than 50 to 60 hz) they audibly vibrate the panels--makes quite a noise!

Thus, I have an interest in reducing or eliminating the resonance from the panels, but DO NOT want to soundproof them. Someone--can't remember who--told me that I could likely stick a 1'x1' piece on the middle of each of the three panels and that would add enough weight to lower the resonance frequency below the audible limit. I just hate to stick a few small pieces on to find it doesn't work and then have to go in and put more around these areas making it look like a patchwork quilt.

I've also heard people say that there are other products out there just as good as Dynamat, and some you can even buy in your local hardware store (I think it was called "brown bread"?). Anyway, just trying to figure out the best solution to my problem, and trying to wrap my head around the theory of how Dynamat (or any similar solutions) actually does what it does. Thanks!

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[CHEMMINS 10+ year member]

Glue some 3/4" mdf to the inside of the panels, and resin the mdf to waterproof it. Done for rattle and flex.

 

[Bettr n' Revrse 5,000+ posts]

Its simple your just reading way to much into it lol...

 

[jonathanengr1 10+ year member]

Will the mdf not block the sound? I have the subs facing towards the front of the boat (subs are in the very back), and of the three sides with aluminum, one of them is directly in front of the subs. I tried facing the subs to the left (towards the side that isn't aluminum), and the vibrations were still there and the subs weren't as easily heard. I could certainly glue mdf to two of the sides, but am concerned it would block the output if I glue it to the side the subs are facing.

 

[Rudy 10+ year member]

I guess it is sort of beside the point since it won't solve your problem but constrained layer dampers work because vibrations in the substrate deform the adhesive. The adhesive is viscoelastic so it returns to its original position more slowly than it was deformed. Strains between the is.

adhesive and constraining layer and the adhesive and substrate dissipate the vibration/movement as heat as the adhesive returns to stasis.

You can't practically add enough mass to a panel to lower its resonant frequency out of the audible range - you need to quadruple mass for every octave. Constrained layer dampers may provide a small amount of stiffening, but not much. Stiffening raises resonant frequency so this and the added mass are mostly offsetting.

In any case, you need to stiffen those panels. They are not only adding distortion by reinforcing the sub's output at their resonant frequencies, they are also stealing output.

 

[jonathanengr1 10+ year member]

So... I don't want to use Dynamat for this purpose? I think that's what you're implying.

If I choose to use mdf, will it block the sub's output if I put it in front of the subs? Or will the sound travel through it? If mdf has the tendency to block the output, should I use 3/4" on the two sides where the sub isn't facing the panels, and a thinner sheet (I think they make 1/4" mdf) in front of the subs? Any idea what type of glue? I assume I'll need to thoroughly spread the glue so every square inch of the panels is glued to the board so there is no vibration against the board.

Also--as mentioned above--I'll use a fiberglass resin to seal and waterproof the boards first (no direct contact with water in this area--just exposure to the atmospheric humidity). BTW--will the fiberglass resin that is sold in Lowes (I think it's made by bondo) work okay? I know it might not be the best, but it's easily accessible.

 

[Rudy 10+ year member]

Have any pictures to show us what this looks like?

 

[jonathanengr1 10+ year member]

Well, I don't have any pictures here, but next time I'm at the lake (this weekend) I'll snap some pictures of the area in question. Tell you what... I might go to the manufacturer's website and grab a copy of the floor plan of the boat and put a note or two on it. Maybe that can suffice for now. Let me see what I can do.

 

[jonathanengr1 10+ year member]

Okay--let's see if this works... the image from the website was tiny, so it's pretty fuzzy. Also, I exported the AutoCAD file to a jpg, and that never looks terribly good. Hopefully everyone can see it all right. Here it is:



The area is in the back of the boat, and is the smaller section of the green area in the image. Looking at the photo, the left, top and right sides are enclosed with aluminum panels, and the bottom section of the enclosure has a vinyl snap-on cover that leads to this area. There is a toilet/changing room in the remainder of the green section.

It seems to be the perfect place for the subs. Since there is a toilet in the area, it is difficult to get to this area (only way to really get to it is to remove the lounge top), so I'd never use it for storage.

Anyway, let me know if this image is helpful and I'll be happy to modify as necessary.

 

[jonathanengr1 10+ year member]

Just posted another pic but it said the post had to be approved by an administrator... maybe it'll be up shortly. I think it's less helpful than the first pics, but it gives you a visual of the area.

 

[pimpnyou204 10+ year member]

The problem from what I've seen is you made the box to fit in another box. Which is ok and all but it's similar to a front fire in a trunk car. Since sound waves go all over and this proves it. Your getting some sound waves going in between the sub box and aluminum box causing the vibration.

So my suggestion and this would probably give u a decent amount more output in the sub is to pull the box flush with the outside of the aluminum box and put more mdf to cover the remaining space between the sub box and aluminum box then caulk or if the space is small enough u can use a foam to fill it in but apply lightly bcuz it will continue to expand. Simply put your gonna wall off the aluminum box like a car trunk.

 

[Phoenix Risen 10+ year member]

am i looking at that wrong or is the sub box surrounded by alum on 5 sides with just the one side open?

if so thats going to be an issue.

 

[i2ain2thunder 10+ year member]

To be honest OP your post TLDR, but inresponse to the title of it: Dynamatt works by increasing the mass of what you apply it too. This creates a change in the resonant frequency of the material (makes it less noisy). It also in general acts as a dampener for sound and insulator. Many people overlook the fact that it is a decent insulator makeing keeping the vehicle cool in the summer and warm in the winter easier.

Read the full post lol, I'd put some MDF in there and sealer up.

 

[jonathanengr1 10+ year member]

Let me modify the pic again... I'll call out which areas are aluminum.

 

[pimpnyou204 10+ year member]

am i looking at that wrong or is the sub box surrounded by alum on 5 sides with just the one side open?
if so thats going to be an issue.

That's what I was getting from the drawing and a very bad idea bcuz aluminum is very flimsy and resonate A LOT!