Light cabinet construction techniques?

[augerpro]

In the link to my webpage I posted earlier, I did this exact XPS foam sandwich. Much better than regular ply, But I actually liked the 1/4" MDF glued together with Weicon Flex 310M Classic better. I need to do harmonic distortion testing to see if there is more benefit to the XPS though (my prediction is no). https://www.somasonus.net/box-construction-methods

[ontariomaximus]

In the link to my webpage I posted earlier, I did this exact XPS foam sandwich. Much better than regular ply, But I actually liked the 1/4" MDF glued together with Weicon Flex 310M Classic better. I need to do harmonic distortion testing to see if there is more benefit to the XPS though (my prediction is no). https://www.somasonus.net/box-construction-methods

Impressive exercise. Can you provide a summary? A lot of graphs to look at.

[augerpro]

No final analysis since work is still ongoing. But the method i just mentioned is the most promising so far IMO. if I were to build something today I would do that with some bracing and melamine or wool batting for lining. Perhaps some of the Resonix CLD tiles attached to the walls.

[Dukk]

Does that spacing still work when the middle of the sandwich holding them together/apart is something somewhat flexible and spongy like that foam?

The I-beam analogy makes sense, but those are using a solid/sturdy material for the "I"'s neck to connect the top/bottom serifs which also adds rigidity along another axis both from the solid connection of two parallels and the shape (like trying to flex a knife along the cutting edge VS wiggling it side-to-side). The foam on the other hand would give a looser connection and a spongy/flabby shape more like a gel or an air gap between the layers instead of a rigid support...at least in my mental picture and inexperience. I think the foam only supports VS sliding the flat plains like a resistance when rubbing your hands together (at least for as long as the glue can hold the fragile foam sides from tearing), while not doing much against other directions of movement or flex.
The foam should offer more resonance deadening compared to a plain air-gap, but it should be possible to get a similar deadening (and equal or superior support) by gluing a few 1/2"x1/2"x1/2" wood blocks between the layers to connect and support the ply while fighting resonance. I think you'd need a solid/stiff connection between the layers for the two layers to get the rigidity benefit of spacing compared to a single layer...might need long+skinny strips or bridge style \I/I\I/I\ dowels connecting the layers rather than blocks to get the strongest effect.
Not sure, more just thinking aloud.
I'm feeling my knowledge gaps pretty hard right now, lol.

I should have some 1/4" ply and some foam insulation on-hand though. Maybe seeing/testing it in person will finally wrap my head around this thing.

Also just realized this sandwich would only realistically make sense if you're using really thin plywood...like 1/8".
If you're planning on using an outter and inner layer of 1/4" ply around a foam sandwich, that would already weigh about the same as simply using a single layer of 1/2"...which is already sturdy enough for touring musicians as a single layer without any added complexity.

To the first bit - Yes. The foam is not flexible and spongy. It is quite rigid for what it is. As long as the top and bottom layer cannot move parallel to one another (shear the foam) then it is viable.

People have also done your thought about lots of little blocks between the two layers. Make the blocks thinner and thinner and interconnect them and you have honeycomb construction.

[LOUT]

Thanks, that makes a lot more sense then.
I might be thinking of the wrong foam (I keep thinking of that closed-cell purple insulation stuff; "Foamular" I think it's called). Or I might be underestimating how firm it is when it has a plywood surface spreading weight and impacts over a larger amount of its surface.

[Dukk]

Sounds like the correct foam. Basic EPS foam insulation. Sure, at 1/2" thickness it 'flexes' over a length but it is a rigid foam.

[augerpro]

People have also done your thought about lots of little blocks between the two layers. Make the blocks thinner and thinner and interconnect them and you have honeycomb construction.

I also did honeycomb construction (NidaCore) in the testing I linked to.

[djg]

In the link to my webpage I posted earlier, I did this exact XPS foam sandwich. Much better than regular ply, But I actually liked the 1/4" MDF glued together with Weicon Flex 310M Classic better. I need to do harmonic distortion testing to see if there is more benefit to the XPS though (my prediction is no). https://www.somasonus.net/box-construction-methods

Apologies if you spoke to this but, how thick a layer of the flex glue? Did you squeeze it out or let a significant layer remain?

[augerpro]

I used a fine trowel that made 1/16" beads and applied the panels like I was laying tile: trowel in one direction and pushed a panel perpendicular to that to collapse the beads. So pretty even thickness for all the constructions except the Nidacore was a little trickier since it it is enclosed in a fabric, so I had to just aim for good coverage.

[djg]

Thank you.

[tvrgeek]

Two layers of 1/4 over 1/2 inch rigid foam will be many times stiffer than 1/2 inch ply.

Touring musicians have different requirements not to be confused with home audio or in-place PA.

Expanding foam is actually pretty rigid. It is SOP for structural panels. What was asked for was ideas on a light weight cab. Well, concentric cardboard tubes with foam meets that request. Not good enough, then some nice 25mm Baltic Birch will be strong enough and suffer the weight. A lot of subs were built with one sono tube. Round is very structurally stiff.

I think this is all being over-analyzed.

[tom_b]

For what it's worth, I think the concentric forms with spray foam web is a great idea.

A structure that contains outer layers with good tensile/compression characteristics and a light weight separator is a stressed skin panel.

[augerpro]

Looking over the original post again, I see this is a sub, somehow I thought this was a fullrange speaker. The methods I posted above aren't really applicable.

[tom_b]

Sure but they are extremely interesting and compelling. I appreciate everything you shared. Thank you.

[tvrgeek]

For those who do not read Russian, it is about the differences between MDF, chip board, OSB and different plywood's.

FWIW, I can't lift my subs any more. (or should not) I could when I was younger. Only 2 cu ft. 3/4 ply with veneer of 1/4 oak ply. Hand trucks are nifty things. Getting ready to build a band-pass sub. Going to make it in two pieces as it needs to reside in a niche above my TV.

[tom_b]

Your point of building it using proven, cost effective, easy, traditional techniques and then using a hand cart are not lost on me. This is probably the best idea in this thread. lol!

[tvrgeek]

If you were going to launch it into space, then the obvious material is aluminum honey comb or aero-gell with carbon skins. Should be do-able for $500,000.

Forgot the old boat builders standby. Balsa core with fiberglass skin, or even the older painted canvas skin. 2 1/8 ply skins with 3/4 inch balsa makes a very very strong panel. You can also make it curved which is even stronger.

[LOUT]

If you were going to launch it into space, then the obvious material is aluminum honey comb or aero-gell with carbon skins. Should be do-able for $500,000.

Forgot the old boat builders standby. Balsa core with fiberglass skin, or even the older painted canvas skin. 2 1/8 ply skins with 3/4 inch balsa makes a very very strong panel. You can also make it curved which is even stronger.

All the talk of sandwich builds made me think of this https://www.youtube.com/watch?v=3Pc-Sd_J26E but I couldn't remember the channel name earlier. Not sure what other kinds of problems something fiberglass and boatlike might have though...maybe none?

[tom_b]

The sandwich talk makes me hungry.