Foam sandwich 15/6 build

[sabre79]

I've been wanting to make a speaker cabinet/s for some time. I wanted either a 2x15 or a 1x15 and a 2x10. I posted on the board to get help about speakers and I got a PM from Mr. Foxen who offered to give me help in using WinISD and coming up with a specific speaker size for specific speakers. All during that time and since then, I've been looking at the fEARFUL threads and looking at the 15/6 cabinet designed by Greenboy.

After looking through several threads on fEARFUL cabs I've started to build a fEARful 15/6 Cabinet. I think it's great that Greenboy has put these designs out there and so many people seem to be happy with their cabinets.

I've read several cabinet build threads (fEARFUL and otherwise) but the build that caught my eye the most was the thread with the fiberglass over foam "composite" build by zac2944 DIY Lightweight Composite Cab Build. I have very little experience with fiberglass so I won't be attempting this type of build.

In the "composite" thread somebody commented saying that they had an abundance of 1/8" luan plywood door skins and they might want to start a build with a foam sandwich panel using 1/8" luan. That gave me an idea. I knew that I didn't want to work with 1/8" luan so I started thinking about 1/4" luan. It is still pretty light. So I've started making panels using 1/4" luan skins with frames made from light redwood and 1/2" foam in the center. Not breaking any ground here, just having some fun.

The sandwich thickness is 7/8" so I had to adjust the dimensions of the cabinet accordingly. The weight of the panels feel much lighter than 3/4" baltic and even feels lighter than 1/2" baltic. The panels seem very stable and light. More to follow...

1/4" Luan on the bottom and light redwood framework with spring handle cutout:


1/2" foam set inside the above frame:


The glued up "sandwich":

[4StringTheorist]

Subscribed!

I'm more likely to go for the fiberglass/foam construction since my wood joinery experience is very slim. This is fascinating stuff, and I love the physics of it.

Good luck! This'll be great to see come together!!

[Petebass]

sabre79, I also built a fibreglass based sandwich cab after seeing Zac's cab.

What sort of foam is that? It looks like expanded Polystyrene, in which case how did you attach the foam to the skins?

[sabre79]

4StringTheorist - Yeah, I love this kind of stuff.

Petebass - I just looked your build up. It came out great.

I used expanded polystyrene and I use Loctite PowerGrab on the foam to plywood and Titebond on the plywood to redwood.

[Foz]

Interesting... if I were going to do something like this - I think I would try to build a full sheet of skin/foam/skin sammich and then cut the parts. Is there some reason this approach is not practical?

[sabre79]

Originally Posted by Foz

Interesting... if I were going to do something like this - I think I would try to build a full sheet of skin/foam/skin sammich and then cut the parts. Is there some reason this approach is not practical?

The reason I'm doing it this way is because It's going to be a mitered joint between the top, bottom, sides and back. I wanted the joint to be wood on wood. I also wanted a wide enough piece of plywood/wood/plywood to glue and screw the baffle in. The wood that I'm using is so light it feels like balsa wood.

Your way would be practical if the plywood and foam were held together with a real strong bond. I don't know if I would attempt it with butt joints.

[sabre79]

Some more in progress shots.


The top, bottom, and sides mitered and ready to be assembled.




Miter held together with spring clamps.




The top, bottom and sides glued and clamped.

[rbonner]

Nice looking project

BOB

[naturalkinds]

That sandwich constructions looks... like a wafer. I'm hungry.

[sabre79]

Bob - Thanks!

NaturalKinds - I was tempted to spread cream cheese over it.


Progress Report:

1. Ordered 6ND410
http://cgi.ebay.com/NEW-Eighteen-Sou...efaultDomain_0

2. Ordered 3015LF
http://www.amazon.com/Eminence-Kappa...2189566&sr=8-1

3. Ordered hardware for cabinet from http://www.reliablehardware.com

This 15/6 will not have an internal crossover. I'm going to get a Carvin BX1500 and use the crossover built into the amp and use the amp in bi-amp mode.

I read in one of the fEARFUL threads about extending the grill down to cover the ports to keep critters out. I'm going to do that here as well.

One piece girdle brace:

[rbonner]

So Sabre, how does the box feel when you pick it up or tap on the sides? Does it have a really solid feel or a sort of hollow light sounding tap / boink?

I like the way you have the wood around the outer edge. I really want to see how this goes. I may want to try some of this technology on the 415 project I'm going to be starting next month. I figure the larger the box, the more weight savings percentage this technology would have.

BOB

[Petebass]

Originally Posted by rbonner

... I figure the larger the box, the more weight savings percentage this technology would have.

BOB

You figure right. My composite 1x12 is slightly larger than the Epifani UL112 but weighs 10.8lbs less (32lbs less 21.2lbs). That's quite a weight saving but on a bigger cab, say, something 410 sized could easily yield a 20lbs weight saving compared to similar commercially built cabs. That's nothing to sneeze at!!

[1n3]

Very interesting so far. My first thought was that two layers of 1/4" plywood = 1/2" ply, but your luan is evidently much less dense than the birch/FPS that's commonly used.

You're obviously an experienced woodworker. Those spring clamps are a new one on me. It looks like the ends are sharp, and dig into the wood to hold. Is that so?

I'm eager to see where this is going.

[Foz]

curiouser and curiouser said alice... whatcha reckon the box weighs?

[sabre79]

Bob - It's very solid when you pick it up and solid sounding when you tap it. I used solid wood on the edges so the joints would be wood on wood. When I am done with the woodwork, I will weigh the box so that I can compare it with 1/2" and 3/4" plywood. I would imagine that a conventional 4x15 would need to be built with at least 3/4" plywood so I think my 7/8" sandwich panels might do just fine.

1n3 - Yes, the spring miter clamps have sharp ends that dig into the wood to pinch the joint tight.

Foz - It doesn't "feel" as light as I had hoped. If it is very close to the weight of a 1/2" cabinet, I might play around with 1/8" luan as the poster who I originally got the idea from mentioned. At least I could make some panels. The other thing is that the glue that I'm using to mate the foam with the plywood is heavy. There are spray adhesives that I could have used that would be lighter. All and all though, I love the solidness of it.

[rpsands]

Would love to see it compared to one in 1/2" arauco. Hope I can get around to building one soon.

[Interceptor]

I very seriously considered constructing building a cabinet using 1/8" wood on a foam core last winter. I dug in pretty deep regarding wood selection, adhesives and spent some serious brain power on running bond strength on different corner approaches.

I'm an engineer by trade.

When I finished up the weight calcs, a 12/6/1 was only going to be a couple of pounds lighter, and a 15/6/1 wasn't even four pounds. This was in comparison to 1/2" birch. The labor and potential fragility wasn't worth it to me. That, and I have no love with dealing with funky epoxies.

My stuff has to survive load out after a couple of stiff drinks in Wisconsin winter. I wasn't sure that a cabinet built with my only average skills would survive. The OP's skills eclipse mine, those are some nice looking panels. The thing that I couldn't get past was how to make the corners sturdy enough. The best compromise was to machine wood pieces as a framework, and then bond panels. It was just at the edge of what I could handle as a woodworker. I wasn't willing to work that hard for the weight loss.

Jim

[will33]

Originally Posted by Interceptor

My stuff has to survive load out after a couple of stiff drinks in Wisconsin winter.

This should be the benchmark of any good cab.

Really, regular butt joints cut straight with good glue should handle whatever a bass cab has to handle. The important bracing spans the breadth of the panels to reduce vibration/lower resonance. That's where the good sound comes from. The short edges of the "ribcage" bracing helps to re-enforce the simple corners. Approach it from the idea of stiffening/reducing panel flex and not your typical structure like a house...or doghouse.

Do it right and it'll still have no problem getting knocked around at 3am in a Wisconsin winter.

I like your approach. If it can't take what happens in harsh environments, all the science and stellar tone in the world isn't going to get someone what they need.

[rbonner]

Originally Posted by will33

The important bracing spans the breadth of the panels to reduce vibration/lower resonance. That's where the good sound comes from.

That's the serious point Will, how does the laminate compare to solid wood.

I was thinking with a box that has to be "approx" 4' X 3' and 20" deep that you could end up with a lot of tinniness to the panels compared to 3/4" Birch. My project the cab is going to be in the 200# range and a guy could really lop off some weight "engineering" panels.

I was thinking balsa wood planks instead of foam core. Either 3/4" or 1" and either 1/4" or 1/8" birch. Luan or birch ply on the inside and counter laminate on the outside could be interesting too.

Balsa is used in fiberglas boats and through commercial sources could be available in large chunks. Just an interesting thought.

BOB

[sabre79]

rpsands - I don't know what my cab weighs but I'm going to bring the bathroom scale to my shop tomorrow. What does a cab in Arauco weigh before speakers, hardware and crossover? I've searched high and low on the board and have not seen it posted.

Interceptor - I think 1/8" Luan could work with some strategically placed wooden ribs inside the sandwich and sufficent bracing on the inside of the cab. I hand-picked light redwood boards for my framework but I think basswood would also work for it is very light. I will most likely build another cab with 1/8" Luan.



Some more shots...

Baffle with 1/2" Baltic speaker mounting rings:




Cabinet with port shelf, girdle brace and mid chamber: