Another Composite cab build - inspired by Zac

[1n3]

This is starting to become a thread hijack, so let me start by saying -
Petebass, I've followed this project with interest and admiration. The recent video is worth a thousand words, as was the earlier photo of you standing on the panel.

Yeah, I was thinking of doing something nuts like using 2" foam or something. It's interesting that the tensile (that's pulling/stretching, right?) is the determiner, so something like ABS might work -- it bends easily but won't stretch very easily at all. One upside is I think it'd be much lighter than fiberglass, and might be more durable to impacts.

Right, tensile strength is resistance to stretching. Compression strength is also important, because when a panel is flexed, the outside skin is in tension; the inside is in compression. The foam core is in both tension and compression.

I looked for a good data set that gave tensile strength for both ABS and FRP, but a 10min google search didn't come up with anything. Based on dissimilar data I saw, I estimate ABS is 1-2 orders of magnitude weaker than FRP in tension. Compression strength would be a little closer.

The 2" foam idea is nuts at all. Since the foam is only about 2lbs/cf, there is very little weight penalty to going thicker, and exponential gains in stiffness. Quoting Petebass, "Doubling the core thickness increases the strength by 4, not 2".

...another idea I had was to cut down on labor by using the ABS on the inside and using 1/8" plywood on the outside. Seems like a possible angle. Is there anything wrong with using different tensile strength materials for skin?

I don't think so... any combination that gives you a sufficiently stiff and light enclosure is good. 1/8" ply and ABS sheet together might make a nice exterior, ala flight cases. Thin ABS can be regarded as a tolex substitute. With some heat applied, you might even be able to work it the same way. But it doesn't have the strength/weight of plywood, let alone FRP. Don't know how well it would work alone in sandwich construction. With a smaller box and thick foam, you might get away with it.

Assuming you've got a plastics supply house nearby, you might stop in some time to see what they've got on hand. Sometimes there are partial sheets you can get cheap.

 

[Petebass]

This is starting to become a thread hijack, so let me start by saying -
Petebass, I've followed this project with interest and admiration. The recently video is worth a thousand words, as was the earlier photo of you standing on the panel.

.........1/8" ply and ABS sheet together might make a nice exterior, ala flight cases. .

Thanks 1n3. I just figured "showing" people is better than "telling" them.

OK this comment here gives me a better idea of what ABS plastic is. That means i can recommend an adhesive - The same West Systems epoxy resin and hardener I used to build this cab! I've since used it to repair my Roadready bass case and can confirm it adheres to it well if you scrape up the area with coarse 60 grit sandpaper first. And we already know it bonds beautifully to XPS foam........

That is in no way a recommendation on strength though. I personally don't think it's be strong enough but i'll be happy to be proven wrong.

 

[1n3]

OK this comment here gives me a better idea of what ABS plastic is. That means i can recommend an adhesive - The same West Systems epoxy resin and hardener I used to build this cab! I've since used it to repair my Roadready bass case...

Yeah, there's a good chance your case has an ABS skin. Not the only material used for that, but a popular one. ABS is everywhere, actually. Many consumer goods have ABS exteriors. Quite a few adhesives work with it, though not all would be compatible with the XPS. Your epoxy would work well, though it's a more expensive option. Non-flammable contact cement would have an advantage if you were trying to form ABS sheet around a corner, since it grabs instantly. If I were going to use ABS sheet, I'd probably try to wrap it like tolex, to reduce the number of exposed edges. Would have to use the thinner stuff, or a lot of heat.

That is in no way a recommendation on strength though. I personally don't think it's be strong enough but i'll be happy to be proven wrong.

I also have my doubts, but I can't say it won't work. You'd be depending more on the strength of the foam core. Again, best odds would be with a small box and thick foam.

 

[44me]

Petebass - a little late here, but congratulations on a really nice build! I've been planning a composite cab based on the fEarful 12/6. I've been trying to resolve a few key construction details, which led me to your post. I've been struggling with whether to include plywood core in the speaker and handle areas. The structure seems pretty easy to crush when tightening the speaker and handle screws. Is this not really something to be concerned with?

Glassing the insides of the side pieces before assembly seems to make things much easier. How many glass layers and what weight material did you use? Once assembled, did you add any corner reinforcements or glass fillets? Did you use any cleats behind the baffle? It looks like the box uses entirely butt joints.

I wonder if constrained layer damping would be effective at damping the mid-frequency resonances? http://www.earsc.com/pdfs/engineering/understandingdamping.pdf Given that your structure is so much stiffer than plywood construction, and the frequencies of concern are reasonably high, this may be a very effective technique. Note in the referenced paper that even covering just a 25% area still results in substantial damping.


Regarding ABS, it has nowhere near the strength of epoxy-fiberglass. If anyone wants to try construction with pre-made skins, consider GPO (polyester-fiberglass) or preferably G10 (epoxy-fiberglass) instead.

 

[Petebass]

I've been struggling with whether to include plywood core in the speaker and handle areas. The structure seems pretty easy to crush when tightening the speaker and handle screws. Is this not really something to be concerned with?

I struggled with the same idea, but abandoned it once the build started. It's not as easy to crush as you'd think. If anything I was more concerned with crushing when I installed the speaker but found I only had to fiberglass around the whole edge and this was no longer a problem. For the handle I just used nice big metal washers (from memory about 3cm in diameter) on the inside of the cab. The bigger the better to spread the load. Sorry i didn't take a pic at the time.

Glassing the insides of the side pieces before assembly seems to make things much easier. How many glass layers and what weight material did you use?

I used a very simple fiberglass weave available just about anywhere which had nothing special about it whatsoever. Based on Zac's build I started off with the intention of using 3 layers of 12Oz. But I ended up using only 2 layers. It was another one of those decisions I made on the fly (of which there were many) which turned out to be the right one.

Once assembled, did you add any corner reinforcements or glass fillets? Did you use any cleats behind the baffle? It looks like the box uses entirely butt joints.

I used butt joints only, no cleats at all, but I did reinforce the corners by filleting a mixture of resin/hardner/high density microbaloons (westsystems 403). This time I did take pics:-

Cab Assembley:-

Close up of the fillets joint reinforcement:-

I wonder if constrained layer damping would be effective at damping the mid-frequency resonances?

I did line my cabinet using foam with an egg-crate like shape. I only lined 3 walls. I did try lining all 5 walls at one point but changed it back to 3... which I'm sure is going to get me a flaming but it's my cab and I designed it for me!

 

[44me]

Petebass, thanks for the additional construction details and pics!

 

[Petebass]

It's been almost a year since I built this composite cab, so it's probably a good time to report back on the cabs durability and overall performance.

The cab still looks brand new! It looks pretty much the same as it does in the pic in the opening post, except for the addition of a "Petebass" badge. Admittedly this has been used as my spare/2nd rig on the basis that the lightest gear stays with me and heavier gear gets carted around by our roadies, but it's still had plenty of use at both gigs and rehearsals where I need to cart my own gear around.

I thought I'd post this to put to rest any fears that this sort of construction wont withstand normal use. I don't recommend letting roadies anywhere near it but like I said, who cares about the weight if someone else is going to carry it for you

 

[5StringPocket]

Great build! How did you attach the corners? I get the use of washers with adequate clearance from corners but what about the fasteners closest to the edges?

 

[Petebass]

Great build! How did you attach the corners? I get the use of washers with adequate clearance from corners but what about the fasteners closest to the edges?

Attaching corners was a real challenge. I tried a few things but the one that worked best was something I stumbled upon by accident while wandering around a hardware store looking for something completely unrelated.

The silly thing is we've used these before as they were intended, i.e to attach paintings etc to walls in places where there is no stud behind the plasterboard. They're called "Wallmates", and they're designed to bite in such a way as to maximise strength in areas of weakness.

I ran some tests on a piece of scrap, and it worked brilliantly. so they made it onto the final cab design.

These ones are rated for 20kg (about 44lbs) each, and I got the nylon ones for 2 reasons. The metal ones weighed more, and I thought the metal ones would be pretty hard to cut down to size. As you can see, they're a little too long out of the box and will penetrate the entire thickness of the panel.

So I simply trim them down with wire cutters.

I'm effectively removing the piece that makes them self-tap into plaster. That's OK because they weren't strong enough to penetrate fiberglass, so I was always going to have to drill holes. I decided to drill 7mm holes as shown,

then fill those holes with hot melt from the hot glue gun. I tested this first to make sure the hot melt wasn't going to burn the foam. Not only did it not burn it, it bonded it to it with rather nicely.

Hot melt hardens pretty quickly, so I had to move fast and screw the wallmate into place:-

A minute or so later, the glue is dry enough to drive a normal 8g screw into the wallmate like so:-

Just to make sure everything fits:-

I'm more than happy with the results! I ended up attaching the grille the same way, and will probably work this into the design of my next cab for attaching as much as possible i.e ports, the speaker itself etc.

There is one thing I would change though. The hot melt works fine on the corners, but because this is an experimental cab I've been removing the grill more often than most people would. The hot melt on the grill attachment has let go on a couple of them and I've had to re-do them. Next time I'll use the same resin/hardner/microballoon filler that I used to assemble the cab on the grill attachment. It will raise costs and be a little trickier, but it'll be much stronger too. But for corners, hot melt glue is working nicely.

 

[5StringPocket]

Thank you so much! Those pictures and explanation are very helpful and that technique makes a lot of sense. I'm not sure I've got the patience for a foam core with fiber glass/epoxy layup but I've really been thinking about a build with NIDA-CORE, H8PP 20MM - POLYESTER SCRIM. The web site lists 0.812 lbs/ft2 and a 48"X84" sheet only costs about $110 plus shipping from Island Water World. I'd be thrilled if I could build the 1212/66 cabinet shown below and come in under 55 lbs. This cab would need about 27 ft2 of Nidacore (22 lbs) which bonds pretty well using butt joints and PL Premium adhesive. I'm curious whether Bondo could be used to backfill and smooth the cut honeycomb edges and apply a Duratex finish to the cab?

 

[Petebass]

To be honest I was quite disappointed with the final weights being reported in the nidacore thread. FWIW I was a fiberglass virgin before this build. If I can do it, anyone can.

 

[Passinwind]

I've really been thinking about a build with NIDA-CORE, H8PP 20MM - POLYESTER SCRIM. It weighs 0.812 lbs/ft2

Have you been following Robby's thread about the grossly misstated weight specs on the Nidacore website?

 

[5StringPocket]

I have been following Robby's thread and waiting for the final outcome before proceeding. Some of the original quoted weights were very unrealistic. If weights approach 1 lb/ft2 you might as well use 1/2" plywood. This foam core with fiberglass and epoxy is intriguing and perhaps the better technique for this application. If possible, I'd like to limit the total sandwich thickness to 1" so as not to exceed 23.5"W x 33.5"H for the outer cab dimensions but I don't know how precise you can be with tolerances using this technique. My other option would be to go with 1/2" Oukume but that would be about a 65 lb cab. 10 or 12 lbs less weight seems worth it when moving this up and down stairs.

 

[Petebass]

I have been following Robby's thread and waiting for the final outcome before proceeding.

Check into that thread again. The final weights are in and Robby has announced the cause of the discrepancy as misquoted weights bythe Nidacore people. The weights published are apparently the weight of the core only without the fiberglass skins. If we did the same for the panels used on the cabs build by Zac and myself, it's no contest. Final weights prove the point.

I would like to limit the total sandwich thickness to 1" so as not to exceed 23"W x 33"H for the outer cab dimensions.

1" foam will be fine for the cab you're thinking about. Zac used 3/4" foam from memory and said he'd only need to go as think as 1" for bigger cabs. I would have done the same but 1" thinck XPS was all I could find locally, and it turned out to be overkill on a cab this size.

If you want to be 100% sure, you can always add a rib stye bracing, also made form foam/fiberglass for extra strength and stiffness. I fact I'd be inclined to build the cab without it as you can always add such bracing later if you decide you need it.

10 or 12 lbs less weight seems worth it when moving this up and down stairs.

ABSOLUTELY!!! In fact the difference between my composite cab and it's wooden twin brother is VERY noticeable, and that's only a 6lbs difference.

 

[Passinwind]

Check into that thread again. The final weights are in and Robby has announced the cause of the discrepancy as misquoted weights bythe Nidacore people. The weights published are apparently the weight of the core only without the fiberglass skins.

Nope.

 

[Petebass]

Nope.

To be honest I dismissed that as damage control spin form Nidacore. I prefer the original version of what went wrong:-
http://www.talkbass.com/forum/f15/c...15-6-1-build-732771/index28.html#post10874722

 

[Passinwind]

To be honest I dismissed that as damage control spin form Nidacore. I prefer the original version of what went wrong:-
http://www.talkbass.com/forum/f15/c...15-6-1-build-732771/index28.html#post10874722

I tend to believe the manufacturer, but it doesn't much matter at this point anyway, eh?

 

[5StringPocket]

Originally Posted by Petebass >
If you want to be 100% sure, you can always add a rib stye bracing, also made form foam/fiberglass for extra strength and stiffness.

I agree. My cab would have single horizontal and vertical wraparound rib braces as shown above, which leaves the largest unbraced dimension at about 14". The horizontal center baffle brace extends 3.5" back which would serve not only to stiffen the baffle but also provide the bottom for the midrange chamber. Side and rear pieces are 2" deep. The bottom port spacer is 3" but vertical and top extension splines are 2" deep. Would 3/4" foam core be adequate for a 33"H x 23"W x 16.5"D box with these wraparound stiffening ribs? I'd think that when laid up with 2 or 3 layers of fiberglass the total sandwich would be about 1" thick. Does this seem reasonable?

 

[deb]

I happen to have a prototype LDS 12/6 (3012LF + Faital W6N8-120) and it's a wonderful sounding cab. Don tested several mid drivers and found that the 3012LF and the Faital mid are a really good match. Your 1212/66 should kill. I'm curious as to what you're planning to do for the crossover.

For Robby's composite cabs, Nidacore was selected mainly because of it's lightness, stiffness, acoustic properties, and ease of use. As far as the weight is concerned, the story is that the published Nidacore weights were incorrect by at least 20%. This has been verified by Robby and the folks at Nidacore. Also, Nidacore may offer LIGHTER weight panels in the near future--as noted in Passinwind's link. Robby mentioned his 3/4" Nidacore 1212/6/1 cab weighs 52 lbs. And that's using the heavier Nidacore panels (0.97 lbs/ft2). If Nidacore is able to supply a lighter weight panel (0.812 lbs/ft2 or lighter!), you might benefit.

Of course, both Zac and Petebass have successfully made ultra-lightweight composite cabs. So, the research has been done--the epoxy fiberglass foam method is tried and tested. You also wouldn't have to worry about edging, which is something you have to consider when working with Nidacore. If you have an aversion to the whole fiberglass side of the composite build, one option would be to look for pre-glassed foam panels.

Good luck with your cab!

 

[Petebass]

Would 3/4" foam core be adequate for a 33"H x 23"W x 16.5"D box with these wraparound stiffening ribs? I'd think that when laid up with 2 or 3 layers of fiberglass the total sandwich would be about 1" thick. Does this seem reasonable?

Actually the fiberglass skins don't increase the thickness of the panels anywhere near as much as you think. I used 2 layers of 12oz glass on each side and the total thickness increase can be measures in millimetres.

3/4 panels with that much bracing should be fine. Just something to keep in mind though - the major advantage of sandwich construction is that adding thickness to the core increases strength significantly without adding any real weight. The stiffness of the panel is proportional to the square of the foam core thickness. In other words, doubling the core thickness (increase of 100%) increases strength by 400%, but only adds about 5% weight. That's a pretty good ratio, a gift even.

So even small increases in core thickness creates significant increases in strength. In the example we're talking and using the thickness/strength ratio mentioned, a 1/4" increase in thickness therefore increases strength by a little over 100%. Unbraced 3/4" in panels are therefore about half as strong as 1" panels. How bracing affects that equation will depend on many factors and I'd personally make up test panels before I could be 100% certain which is stronger.