Foam core construction for subwoofers

[agent.5]

Just to clarify: are you guys talking about pvc foam core here? i assume the high density version. Something like this (no affilation with seller):

http://www.boatbuildercentral.com/pr...0Plain%20Sheet

what do you guys think about honeycomb core? for example,

http://www.boatbuildercentral.com/pr...id=30&NIDACORE

[morbo]

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?

[mwmkravchenko]

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]

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?

[mwmkravchenko]

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

[justinc]

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

[454Casull]

You can use aluminum honeycomb instead of foam.

[agent.5]

Will these foam work? The 1" is priced pretty good.

http://knoxfoam.com/cgi-bin/SoftCart...402+1124955649

[mwmkravchenko]

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

[morbo]

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!