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Lightweight cab alternative construction idea

muddycreek

muddycreek

Feb 26, 2010
352
240
#1
OK, so poking through a lot of the threads here, from the fearful cabs to Zack's foam/composite boxes (which I'd love to take a shot at).

So, I'm here with a crapload of 1/8" lauan plywood, and it occurs to me- it seems I should be able to get a reasonable stiffness out of a lauan/foam/lauan sandwich, especially if I went to a 1 or 1 1/2" depth. If I edged each panel inside the lauan (kind of like a foamcore door) with a half or three quarter inch strip of something moderately light and stiff, like Lauan, I'd be able to glue and screw the whole thing.

Sure, not quite as impact resistant as solid ply, but that's not a big concern for me- and if it was, I could even Formica the whole thing and put on some sort of corner protection, and I bet it'd still be noticeably lighter than braced 1/2."

Anyone ever tried this? Anyone know the differences between a well bonded skin of fiberglass and one of lauan? Is the main worry just impact resistance, or a real difference in rigidity?
 
#4
This is an intriguing idea. I speak from 29 years of professional woodworking, in addition to being a bass manufacturer.

First, I'd stay away from screws which will distort your material. Use glue unless you absolutely must use mechanical fasteners.

Second, I'd use an adhesive, foam to lauan. Then glue your frame pieces, which will be with a different adhesive.

Third, if you have a good tablesaw, you could miter the panel to panel joints and get excellent glue joints there--superior to plywood to plywood miters.

Clamping is best with band clamps for even, gentle pressure, You'd have to do four sides at once, but a glue like Titebond II Extend would allow you the open time to get all the stuff put together and clamped.

The bigger the enclosure, the greater the savings in weight (and, I suppose, the more likely the panels are to vibrate).

Careful planning for handles and jack panel will be an early requirement.

I'm subscribing to this thread to hear all the input. This could be an interesting and worthwhile discussion.
 
#5
Hi.

Any kind of sandwich construction is interesting from the science point of view.

90% of the DIY sandwich constructions (regardless of the application) fall on their ***es on the most common goal = weight reduction.

Back in the day in the poly, we made several attempts to recreate aluminium honeycomb sheets used in aircraft industry. There was several different methods we tried, including plywood and foam.
Everything was relatively easy up until the need to join two pieces together. The added weight of the stiff enough joint made the experiments to fail, every single time.
When design parameters were changed from weight to some other parameter, the results got better.

By all means go for it, it's an interesting experiment.

Regards
Sam
 
#6
I was thinking a lot along similar lines- keeping the weight down will be interesting. I figure I've got two choices- relying on glue joints where all the lauan pieces meet and keeping the foam core continuous, or somehow blocking the edges and adding weight.

To Lee- I was thinking I would not go with a frame, or at least a separate one. If I use a light and stiff enough wood, I can run a 1/2" wide edge all around the panel inside the lauan skin. That way, I can glue and screw the edges and not rely entirely on adhesive and clamping. Poplar's my current thought, although I have some eucalyptus, which I may use just because it's here, moderately light, already 1", and very workable.

Have to pick drivers first, though. Probably a couple 15" Deltalites and a mid, but I'm still wrestling with myself over what I want.
 
#7
A better glue would be PL Premium construction adhesive. It expands as it cures and will fill almost a 1/4" gap. It's great for edges that aren't perfect.

Wear nitrile gloves because it sticks to everything, including your tools. When it dries to a rubbery consistency, you can easily scrape off the excess from the outside of the cab. Once it's hard, it would be tough to chisel it off and not harm the thin lauan veneer.

You'll probably need interior bracing in many areas. This generally has to be solid wood, which will bump the weight a bit.

Consider how you will tighten the speaker enough to avoid air leaks. Those bolts need to be very tight, and they might pull through the lauan.

Just things that come to mind.
 
#8
Good point- I was thinking about the baffle, assuming I'd do one of two things: embed a circle of solid wood in the sandwich so I could rout out the speaker holes and be left with a solid 1" or so ring around the cutout, or just do the baffle with braced 1/2".

Jon
 
#10
This is all good. Clarifying, when I said "frame" I meant a perimeter piece around the foam, giving the four edges of the sandwich a wood surface. Sounds like vocabulary got in the way of identical thoughts!

Rick, is that a blown polyurethane adhesive? Your counsel fits that kind of product. If the miters are good, the cleanup is extra work. If not, that may be a good idea. When I hear "construction adhesive" I think of the solventy stuff in caulking gun cartridges, and I don't think that is a good application here.

The internal backing (if that's not a contradiction) for the speaker hole is very good. I had envisioned going with 12mm Baltic for that panel, but staying with the experimental sandwich for that part of the prototype seems better in several ways.

Exciting to ponder!
 
#11
Lee, Link Removed is a tube-based adhesive with low VOC; I've used it indoors with no odor problems. Naturally, there's no need to clean the interior seams; the PL makes a great fillet. Removing the excess when it's rubbery was easy with a small metal paint scraper. When it's hard, it sands fairly well.

I clamped some cabinet joints during curing. Others, especially the braces, were held with brads. There's no need to remove the fasteners. If you use drywall screws on outer cabinet corners, it's best to remove them after the glue cures, so you can rout the edges.

A technique used by many of us who have built Bill Fitzmaurice's cabs is to keep one hand gloved, the other bare. This allows you to handle tools with the bare/clean hand. With both hands gloved, you never know whether there's PL Premium on there, and it ends up on your tools!

Removing PL Premium from the hands is best with wet paper towels before it dries. After it dries, it takes quite a bit of time to wear off!

PL Premium provides plenty of working time.

(I'm being careful to always include the "Premium," because there's a non-premium product too.)
 
#12
Rick Auricchio said:
Depending on how much baffle there is outside the speaker, it may be simpler to just use 1/2" plywood. Consider how much labor it will take to build that special sandwich.
Click to expand...

But that's the fun part! I was going to use the same 'hide a chunk of wood then cut it mostly out' for the rear panel, and possibly handles- I don't think strap handles (my first choice) are going to be weight-effective when you add enough blocking inside the sandwich to support them. Although I was thinking about some minimal inside-the-sandwich bracing anyway, and I could put strap handles on that. But then you're back to adding wood weight. Maybe I'll just wrap my arms around it and carry it.

I had thought about PL as an adhesive, and may just use it where it's most likely that I won't be super-flush, like around the circle cutout on the front baffle (if that's the way it happens). I'm a bit leery about the expansion idea- if something's clamped or screwed, I suppose it's OK. I guess I'm envisioning expansion foam type expanding, but I suppose PL won't actually expand enough to bow things.
 
#15
Rick Auricchio said:
The expansion of the PL just oozes out of the joint. The ooze lets you know you've applied enough. It doesn't expand like spray foam.
Click to expand...

Thanks. I hoped somebody could answer that.

Now I need to pick format and drivers. I'm playing through a fairly terrible sounding FrankenPeavey 410TX and need to step it up a bit, maybe some interest in my little experiment will provide me with some much needed peer pressure to get it done.

So everyone can either learn something or point and laugh.

Jon
 
#16
My experience with blown poly is there has to be meaningful--not reefed down, just meaningful--clampage. I have made mitered wood containers by using masking tape hinge/clamps on the dry fit, opening the thing up, applying yellow glue, and then rolling it up and closing it with tape and it has held fine.

Sounds like the PL Premium is something I've got to get in my arsenal. I'm interested.

I would think a small block where each machine screw of the strap handle goes would not add much weight, all glued in and all. Bore the hole and install a teenut from the inside.
 
#17
Lee Barker said:
I would think a small block where each machine screw of the strap handle goes would not add much weight, all glued in and all. Bore the hole and install a teenut from the inside.
Click to expand...

I thought about that, but with a 1/8" lauan skin, it'd have to be more than a small block- I'm rapidly giving up on the 215 idea in my head, but even a 1-15 is going to be a lot of weight to put on a small wood block trying to pull itself through the outer skin. I've thought about LONG 1" tubular webbing straps acting as handles that go corner to corner down both sides, attaching to the 'hidden frame' at the corners instead of out in the middle of the sandwich.

Dunno, I'll roll it around in my head a bit more. First thing is to go buy the $12 sheet of foam and glue up some strips to stand on and abuse- I've got ten or twelve full door skins.
 
#18
A larger block, say 1/2 x 4 x 6, would distribute the force over a wide area. It would weigh a few ounces but really do the trick.

To choose a driver, your best bet is to model a few neo drivers with WinISD. Pretend your box is made of wood. The resulting model should give you a box and port size, and you'll have some figures for frequency response, max SPL, and power at fart-out.

Lee, using woodworkers' glue requires you to have nice tight joints. With PL, you can even make somewhat sloppy cuts (like with a handsaw!) and it fills all the voids. Certainly no need to miter. The butt joints, once cured, are stronger than the wood. Then you just clean 'em up on the outside with router/sander.
 
#19
Yes, I've been mucking around in WinISD (which is kind of a pain, because it's on the seldom-used windows partition on my mac). I'm mostly settled on Deltalites or Basslites, depending on the configuration and eventual impedence of the cab- I don't have the power to push Kappas, it's a lightweight cab so NOT using neos seems stupid, and nothing else models as well or enjoys the same reputation for the price. I just need to decide the configuration and commit in my own little brain.
 
#20
25 years ago we used 3/4" thick aluminum honeycomb (both faces and core) for some structural projects.
They also had raw paper and plastic honeycomb cores that would lend itself for a core material between wood faces. I'm sure the technology is far better now than then although I wonder how much a paper core would cost in todays market.
 

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