Anything between "doubled" MDF?

I'm building a sealed sub using an Ascendant Audio Atlas 15 and was planning to use double sheets of 3/4 inch ( 18 mm ) MDF for all six sides. Is there any advantage of putting any material between the MDF sheets, or simple glue and clamp them? Or if someone would please direct me to anywhere on this forum or web that might discuss this?
 

Raoul

Member
2003-02-03 8:39 am
OR
If you search for "sand sandwich" you will find a few ideas on mixed material enclosures. I have a Brahma 12" in a ported box, constructed of 3/4" MDF with a layer of 1/2" MDF glued to the outside. The result is a very dead box. Just be sure to clamp the two layers together well, or use a lot of weight to hold the layers together as the glue dries (this was my method).
 
DO NOT use anything other than hard glue between wood sheets for a sub enclosure. The above suggestions for contrained layer damping are fine and dandy (and effective) for full range enclosures, but ARE NOT appropriate for sub enclosures.

For the low frequencies involved in bass reproduction, it is highly unlikely (in a well designed enclosure at least) for any panel (side) mode to be within the operating range of the sub (i.e., they will all resonate much higher than the range the sub plays in).

If you aren't looking at resonance problems (and in sub enclosures you are not, as I explained above), then damping is of little significance. Your most effective approach is to push resonance as high as possible and also push stiffness as high as possible (the two aren't necessarily a direct relationship). Stiffer panels result in lower forced vibration in non-resonant frequency ranges (i.e., they flex less under the cyclic pressure loading inside a sub enclosure).

For a flat panel, stiffness increases with the cube of thickness. a 1.5" wall is eight times stiffer than a .75" wall. This is only true if the layers making up the total thickness are rigidly bonded and act as a single layer. If you have a soft layer between, then you effectively have two separate .75" layers, which means the wall is only twice as stiff as a single .75" layer. Using a soft glue, a layer of damping material, sand, or other "constrained layer" approach will result in a combined panel that is only 1/4th the stiffnes you could have by using simple wood glue.

As far as reducing any higher frequency panel ringing (resonation) induced by distortion and/or external sounds, a simple internal layer of damping material is more than sufficient if the panels are already stiff. There typically isn't a need for that. Don't forget bracing though... dividing a panel is very effective in increasing its stiffness, and for a given enclosure weight one highly braced will be stiffer than one less braced but with thicker walls.
 

roddyama

Ex-Moderator
2002-01-19 9:25 am
Michigan
RHosch said:
DO NOT use anything other than hard glue between wood sheets for a sub enclosure. The above suggestions for contrained layer damping are fine and dandy (and effective) for full range enclosures, but ARE NOT appropriate for sub enclosures.

For the low frequencies involved in bass reproduction, it is highly unlikely (in a well designed enclosure at least) for any panel (side) mode to be within the operating range of the sub (i.e., they will all resonate much higher than the range the sub plays in).

If you aren't looking at resonance problems (and in sub enclosures you are not, as I explained above), then damping is of little significance. Your most effective approach is to push resonance as high as possible and also push stiffness as high as possible (the two aren't necessarily a direct relationship). Stiffer panels result in lower forced vibration in non-resonant frequency ranges (i.e., they flex less under the cyclic pressure loading inside a sub enclosure).

For a flat panel, stiffness increases with the cube of thickness. a 1.5" wall is eight times stiffer than a .75" wall. This is only true if the layers making up the total thickness are rigidly bonded and act as a single layer. If you have a soft layer between, then you effectively have two separate .75" layers, which means the wall is only twice as stiff as a single .75" layer. Using a soft glue, a layer of damping material, sand, or other "constrained layer" approach will result in a combined panel that is only 1/4th the stiffnes you could have by using simple wood glue.

As far as reducing any higher frequency panel ringing (resonation) induced by distortion and/or external sounds, a simple internal layer of damping material is more than sufficient if the panels are already stiff. There typically isn't a need for that. Don't forget bracing though... dividing a panel is very effective in increasing its stiffness, and for a given enclosure weight one highly braced will be stiffer than one less braced but with thicker walls.
Right On! :up:
 
RHosch said:
DO NOT use anything other than hard glue between wood sheets for a sub enclosure. The above suggestions for contrained layer damping are fine and dandy (and effective) for full range enclosures, but ARE NOT appropriate for sub enclosures.

First of all, I want to thank everyone for your feedback! For this sub, I plan on just using glue, but the other input definitely gives me something to study for my future projects.


RHosch,

Just to make sure I don't missunderstand and do something stupid, what exactly do you mean by "hard glue"? Titebond I, II, III? Gorila glue? Elmer's Carpenter's Wood Glue? others?

Thanks again!

Tommak
 
tommak said:


RHosch,

Just to make sure I don't missunderstand and do something stupid, what exactly do you mean by "hard glue"? Titebond I, II, III? Gorila glue? Elmer's Carpenter's Wood Glue? others?

Thanks again!

Tommak

Any wood glue would be a "hard" glue. Titebond and Elmers for specific brands, Urea based resins, epoxies, vinylesters, PVC's etc. for generics. Even glues like Gorilla Glue that retain a little elasticity would be fine since the bond acts over a pretty large surface area.

To be a little more helpful, I'd say avoid any glue that requires a thick bondline to be left. Although some of those do harden, many don't and there are plenty of alternatives that you just "clamp tight, squeeze hard, and forget." :)
 
I question the use of any glue that "dries" for gluing panels together. The only time I've done a double panel speaker, I first tested yellow wood glue with 2 1ft X 1ft pieces of plywood. I let it dry for 3 days with pressure. I was able to pry the panels apart to expose wet glue. Sure
the edges were dry, but not the center. Instead I used epoxy which cures. How is a glue that "dries" supposed to do so when sealed inside of 2 panels?
 

Raoul

Member
2003-02-03 8:39 am
OR
Hi johninCR,

That's an interesting observation. I have glued layers of MDF together with carpenter's glue and never had that problem. I wonder if plywood is less absorbent than MDF. I smear an even coat on the panel, then comb it with a V-notch trowel. After a couple days with a bunch of cinder blocks piled on top, the panels are very secure and well bonded. How much glue did you apply between the layers?
 
what about liquid nails? I like the "heavy duty" variant for cabinet construction as it bonds the MDF together and provides an airtite seal (I spread the stuff that smooshes out of the join around th bond inside. If I don't have enough I'd seal with clear DAP ALEX plus). I use the liquid nails along with 1 1/2 inch finishing nails to hold my cabinets together (subs and full range speakers) and I have found it to produce very strong boxes. I have had experience bonding large surfaces with it and it has always dried. but I'm not sure how well it could bond two 2-foot squares of MDF. If I were to construct a "doubled" sub box, I would first build the entire "single" box, and then attatch the second pannels on to it. I would change around the box so that a face of the box with seams(I use simple butt joints) will be completely covered by the second board. I would apply the glue/epoxy to the boards and then nail them on. I would nail around the perimete like I usually do (nail about ever 1 to 3 inches, depending on box size and bass output), and then nail around on the inside of the new pannel to make sure the two pannels are well connected.

I have never done a "doubled" sub box and my woodworking skills are not that great, but this is what I would have done if I were to make a new killer sub.