Foam core construction for subwoofers

I am soon going to be building what I *hope* will be the last subs I ever build. To this end, I want to make the enclosures as stiff as possible, since below 100hz stiffness is the most important characteristic of an enclosure. I am currently planning on using 1" plywood, braced extensively. Weight is a bigger constraint for me than volume, these are ~2 cu. ft enclosures, and I have a fair bit of breathing room (max external dimensions are 23x18x16"). I came across this article in searchign for ultrastiff materials to build from or reinforce with, and am curious as to whether folks here agree with the conclusions about rigidity. I know CLD techniques improve damping a great deal, but I have not heard these arguments re: the resulting rigidity before.

http://boatdesign.net/articles/foam-core/

foam-core-2.gif
 
Foam is not all hot air

I did some work like this some years ago on the front doors of a couple of churches. To drop the weight I sandwiched the panels with closed cell foam. Worked like charm.

If I were to apply this to a driver box I would laminate a foam core with some thin plywood. If cost is no object then I would use 1/8" baltic birch. The other thing to keep in mind is that a CLD construction is stronger when applied to curved panels rather than flat panels. What you need to pull that off is a decent vacuum press. It would allow the type of clamping that you would require.

Mark
 
re materials

This is a good method of making enclosures.
Basically the fundamental resonance is proportional to the square root of the panel stiffness divided by the mass per unit area.
In things like mdf and most plastics this ratio is about the same and so is that of steel, the difference being that steel is about 100 times stiffer, so the amplitude of the panel vibration is around 40db. less.
Materials with foam or honeycomb cores push the panels resonances up to much higher frequencies, the ultimate material is probably carbon fibre honeycomb in this respect.
Tests done by Small showed that the sound pressure inside a closed box decreases approximatelly hyperbolicaly with frequency, so cored materials can push the principle panel resonance up to frequencies which have very low driving powers and are well above the subwoofer range.
 
Foam is your friend. When we constructed the hull for our University's Human Powered Submarine, we did a sandwich of fiberglass, 2x carbon fiber, and fiberglass, then a layer of 1/8" foam, followed by another F, 2x CF, F. The resulting hull was insanely stiff compared to a similair layup w/o the foam. You could jump up and down on a 1' square panel layed across a fulcrum w/o any flex.
 

Ron E

Member
2002-06-27 10:41 pm
USA, MN
I have thought about doing this since I saw how strong composite materials can be. At school we had a fiberglas/aluminum honeycomb material that was about 1/4-3/8 inch thick and a ~12" long piece could be supported between two objects and hold over 300 lb with very little deflection. The trouble is how to join the panels.

Technically, this is not constrained layer damping, the foam is merely acting like the web in an I-beam, moving the strong parts (the skin) out to where it does the most good. Most stiff foams do not have the greatest dampoing properties - there have been some experiments with styrofoam cones and the breakup was pretty bad....including the oval/rectangular KEF B139, IIRC.

One possible issue with very light subwoofer enclosures is reaction forces, especially with PR subwoofers. The motion of the woofer can be enough to shake the entire enclosure and make it rattle or walk across the floor.
 
Quick idea

See Attached sketch. The brown skin is thin ply or your other idea of a fiberglass or carbon fiber skin. The corners are of course corner blocks. The woofer cut out would have to have a ring of wood to. But it could work. The foam could be laid up in segments as a brick layup. Then with the appropriate hot wire saw you could cut it to shape. Then you must laminate the interior and the exterior. I might just do this to see how it sounds. I've always made my best boxes like a battleship. Maybe it's time to float like a butterfly:D

MArk
 

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Carbon fiber would be a bit overkill in this case, or very cost prohibitive at the least. We used it on the sub because we had it donated to us by Fiberlay who I will shamelessly plug for having great service and good products. They have a wonderful two part epoxie for wet layup work that yields great results at ambient temperature and pressure when done right (although it is rather toxic, so a resperator is an absolute must).
 
I use to work for an exterior home improvement company and they used a material exactly as you are describing for the sunrooms they built. I think they had some aluminum joints that held it together. The material was very very strong and very very light. It was pretty thick tho(about 2 inches I'd say).

Reactance forces from the inertia of the cone moving could be easily avoided with Push Push driver mounting(opposing drivers to cancel out forces)
 
Re: Foam is not all hot air

mwmkravchenko said:
If I were to apply this to a driver box I would laminate a foam core with some thin plywood. If cost is no object then I would use 1/8" baltic birch. The other thing to keep in mind is that a CLD construction is stronger when applied to curved panels rather than flat panels. What you need to pull that off is a decent vacuum press. It would allow the type of clamping that you would require.

Actually, you shouldn't need a vacuum press to curve the panels. If I understood the technique properly, you make cuts in the material before applying the bonding resin. If you adjust the density of these cuts, you can use them as a basis for bending the plates, just like the patented process used by a certain Danish cabinet company.

HTH
 
Thanks for the interesting discussion everyone! Lots of good points here...

Its interesting that the composite material is stronger if curved, I presume this is because it is easier to start compressing/tensioning than to push something already compressed/tensed further?

Also, I just want to be clear, in this type of application, are we talking a stiff foam, like the pink type that is sold for wall insulation? This type of foam is like a stiffer version of styrofoam, and will snap if bent far enough. Or, are we talking about a flexible foam like carpet underlayment or regular grey closed cell foam, the type of foam you could roll up?
 
It's Not just a bunch of HOT AIR

THe foam should be a high density closed cell foam. Like the stuff used to sheath buildings. Or if you have a source the kind used in boat building. But between the types there is not a huge difference in structural properties.

Concerning the kerf cut bending scheme. I would stay away from this if I could. A cut in a panel seriously compromises the strength that is available. But a thinner in tact panel when bonded to the foam core and the other panel on the opposite side will be very strong. It will bend farther without snapping in to little pieces. Mostly because it will be much more springy. Higher modulus of elastisity. ( Or lower? Heck I can't really remember it's getting late. )

Honeycomb panels are really good to. They are harder to make into a curve but it can be done with flats as in a conventional box. I've used cardboard honeycomb in larger cabinet projects to save weight. It is quite stiff. Just check out the doors in you home and you most likely have a good example of a honeycomb panel in everyday use.

The curved shape is better to work with. Just think about how many square pressure tanks you can come up with! Round rules!

Mark
 

Ron E

Member
2002-06-27 10:41 pm
USA, MN
If you are going to use a curved panel, there is no need for foam core, you can just use a sonotube and dispense with all the expense and trouble. If one really wants a composite curved structure, why not use two different diameter sonotubes and fill the middle with low expansion foam?
 
It's hard to argue with you when you make sense!

If you are going to use a curved panel, there is no need for foam core, you can just use a sonotube and dispense with all the expense and trouble. If one really wants a composite curved structure, why not use two different diameter sonotubes and fill the middle with low expansion foam?

Yep!

Mark
 
If you were to cut the sonotube in half, to make a half round, and maybe attatch it somehow to a backless rectangular cabinet, would the sonotube be strong enough then alone? or would you need to use the two different sizes filled with foam inbetween as suggested above. say for subwoofer use
 
It would be great for a padded room

That type of foam is not rigid. It's designed for upholstery. The types of foam that have been discussed are all rigid foams. They are used in the construction industry for various things. Some of them are very rigid such as the urethane foams used in boat building. The house sheeting is also quite rigid and considerable cheaper.

Mark
 
After pricing the foam and thinner plywood sheets, I've come to the conclusion that in my particular application, this would be no cheaper, and be significantly more labour intensive than just using standard BB ply. I will definately be investigating it in the future though, for curved enclosures and maybe together with CLD techniques in a full range speaker. Thanks for all the good ideas here!