This has been an interesting use of lockdown time, doing without power tools for once and using a sharp knife to cut foamboard into 0.53x Karlsonators for a pair of Tang Band W5-2143 full range drivers.

Given this was my first foamboard project, I made a few boo-boos but none so bad I couldn't cut and glue my way back again.
In the end, it doesn't matter much if you have to start again; you get better as time goes by.
Stupidly, I ran out of hot melt glue at a critical point; didn't realise at the time that the extras I was sold for the gluegun were the wrong size.
Anyway, Jaycar came to the rescue during lockdown; got so many i will never run out again.
Holt melt glue is great for most of the bits you need to cobble together, but you need to be real quick applying glue and getting the foamboard in place, or else it dries too soon
and you have to start again. So for the more difficult bits I used Ados. For the final side panel I will use Gorilla glue to expand into any of the little areas that might create
pinhole leaks.
I opted to use 1cm foamboard but most builds by experts use the 0.5cm stuff which is much cheaper but you need to add lots of fillets and braces with offcuts to ensure adequate panel rigidity.
The only concession to bracing I've really bothered with is for the speaker baffle; instead of doubling up on foamboard, I glued hardboard to the back as the drivers are quite solid. And that way there's something for the screws to bite into.
These things work out to be superlight; foamboard weighs next to nothing.
Looking forward to see how they sound; the W5-2143s are great little inexpensive full-range drivers. Probably my faves to date.
In this enclosure they're meant to bottom out at around 50Hz in room.
One final thing; the plans are for 12 and 16mm materials; you will need to make a few measurement changes if using 10mm foamboard. But essentially it is just a bit smaller all round; the interior dimensions are much the same.

Hi Luigi,

Looks interesting but what is foamboard ?

Greetings luigi!

The original 1950s Karlson speaker design was a strange critter indeed....

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Aug 25, 2020 16:49:20 GMT 12 cartridgeguyonline said:

Looks interesting but what is foamboard ?

Hi CGOL,

Good question. It is essentially EVA foam sandwiched between two layers of paper. 
You can buy it from art supply stores, and kids use it at school for building projects.
I used the 1cm stuff which is way more expensive than the 0.5cm material that most seem to use over on the DIYaudio forum.
But it requires less bracing. However, it is harder to cut evenly so easier to get air leaks. You need to have no airleaks 
to make the bass work properly. The design, if memory serves, is a 6th order bandpass. 
The unusual front baffle is called a Karlson slot. The speaker doesn't really work properly without it, at least so they say.
Will be able to comment after this weekend when it should be finished.

The whole idea behind foamboard constructions is a quick and dirty speaker experiment.
Because I used 1cm foamboard it has been anything but quick, and really quite dirty. 
If it works, the idea is that you build one of wood. However, many have tried and been disappointed by the results.
If you build it in wood, you need to ensure you use plenty of felt in the right places, especially behind the driver and lining the outside of the baffle the speaker attaches to.
Those made of foamboard aren't so critical, apparently. I guess the foam works as a kind of absorptive material.
You can even make these from EVA foam if you want, though it is usually more expensive to buy than foamboard, and not as rigid. 

Forgot to mention. The other reason people have taken to foamboard for speaker construction is that you don't need cutty powertools
with rapidly spinning blades. You can just cut off your finger with an Exacto knife instead.

Straight lines through foamboard aren't so hard using a straight edge of some sort. You just need to be mindful of keeping the knife at 90 degrees to the foamboard. And you do get better at it with experience.

It's funny producing a speaker that weighs little more than the driver though. With this design, the driver itself is mounted low and in the middle of the box so it shouldn't jump around too much, providing it isn't sitting on a slippy surface.

The idea behind the Karlsonator is that it is quite dynamic and even smaller units can fill a room quite well. Mine's designed to go down low enough to reproduce a kickdrum well. Guess I will find out sometime soon.

If it doesn't work out, I can always go back to what the Tang Bands have been in up until now, a transmission line design within a cylinder.

Thanks Luigi,

And once I saw the photo of the wooden I realised that I once had a pair. Admittedly without drivers so never got to hear what they were like. I got them off an old chap in ChCh who had expired. I thought they were horns of some type at the time. I think the fellow I sold them to got them running with Tannoy drivers iirc.

I have never used foamboard but have used uPVC foam for various things about the place. Its nice to work with but doesnt like being left in the sun as it buckles.

Is that the same as the protective foam that things like amps, speakers etc are nestled in to prevent damage in transit?

If so, you can use that stuff to build compact subwoofers, like so:

www.diyaudio.com/forums/subwoofers/258433-light-air-slot-loaded-band-pass-sub.html

Looks like you could run one of these up in an afternoon!

No ive not seen uPVC foam used for packaging though the way some of the couriers treat stuff maybe they should ! Most foam packaging is either the polystyrene or Polyethylene stuff. uPVC foam you can get from plastic suppliers and its quite dense and solid. I usually get them to precut mine as well but it cuts and drills ok with normal woodworking tools. Would be a good option for speakers I think. Must try it when I get flush.

Yes, you're right it might well be okay. Not that cheap, but; the foamcore guys would scoff. They pay a dollar a sheet from the dollar shop in the US!
Like this then:

cambrianplastics.co.nz/product/pvc-rigid-grey-standard-sheets/

So I went in today and had a bit of a mosey around Modern Plastics in Greenlane and their sheets of rigid PVC are pretty expensive.
About the best stuff that looked like it might work okay was called Celuka.
In 10mm sheets of something like 1.22x2.44m, it costs nearly $200. But they can also get sheets the same size of 0.6cm thickness for around $130, and a sheet that big would make a couple of sets of standmount speakers, I'd imagine. A nice finish too.
This stuff you can cut by knife as well. And it is really quite rigid.

Cool stuff,

I've been eyeing up these designs for a while.
Keep up the good work, I look forward to your results.

Cambrian Plastics are the other established plastics supplier in Akld, out west.

Consider also Corflute - however, to handle bass energy, I suspect you'll need not only stiffness but also more mass.

Yes, keep us posted.

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Corflute no good because the edges are honeycombed so you need to plug the gaps which is a pain. Otherwise, the glue just runs straight down the drainage holes!

I finished the Karlsonators over the weekend. They are stupidly good for what they are; bass output in particular is robust, especially so with the front slot panel in place.
The bass is also very articulate. No need for the TLine subwoofer with these in action. Guess the speaker being close to the ground helps. So too does the 6th order bandpass design.
However, I need to do a bit of tweaking to the slot because they sound a bit closed in - there are several workarounds for that.

The idea of the slot, aside from running the backwave over the frontwave, is to broaden the image and it does that for sure.
All in all, a great way to get the most out of a full range driver. The W5-2143 is really a very good thing, especially at its price.
It's at least the equal of Mark Audio Alpair 10P.
I tried to upload a photo of finished item but it was in an unusual format, so this is them without the slot attached.

The Karlson slot reminds me a bit of Voigt's tapered pipe.

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Fwiw, the originals I once briefly had were a lot larger than those and had a curtain of grill cloth hanging inside so you couldnt actually see the speaker.

The purpose of the slot is to increase the hf output. It in effect reduces the size of the cone as the listener sees it so it is closer to the diameter of a tweeter horizontally. You can demonstrate this effect with say an 8 inch speaker by taping two pieces of wood either side leaving just a narrow slot. The hf will increase at least in the horizontal plane. It won't do much for the bass however and I guess that is why the Karlson slot is shaped the way that it is. I have no idea how well it works in practice but suspect that the shape of the slot might vary with different drivers. I've seen it used with a 12 inch Tannoy which doesn't really need a hf boost apart from over (for MGs) 15khz. Murray Dick's ESL speakers also had an oval shaped slot which I hesitate to describe.

So I must have changed the setting on my camera phone but here is a shot of them with slots minimally attached.
I get the feeling that with this design you can mix and match drivers a little, just like with the original Frugal horn .
So long as it is a 4-inch full ranger, anything with that description will work, at least to a degree. With this 0.53 Karlsonator, 
many five-inch full rangers can be used to good effect.

The slot is also to modify imaging; instead of for one person, the idea is that imaging is across a wide area, and you can even walk around the room and it doesn't change much.
However, as it was it sounded a bit closed in to me. So initially, I removed the slot and put a spacer in place, with the driver sticking out about 10cm from the baffle. This opened it up really well, but the driver was a bit vulnerable so returned it to the original design and added a couple of slots on the sides. This has worked well.
Will now modify the Karlson slot so that I can see all of the driver, and open up the top part a bit too.
There are all kinds of Karlson slot shapes so it really doesn't matter what you do in the scheme of things, so long as the basic shape is there.

Next project is a KaZBa, open baffle subwoofer, dual driver. No foamcore for this one, but a real easy build. If it doesn't work, then plan B is an MLTL with a 15-inch Dayton sub.

Good work so far.
I must say that I'm surprised that the lightweight material will handle LF energy & simulate a wood cabinet in that regard.
However, I have played with this sort of foam material for many years (20?).

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The most critical factor seems to be the front wings. With bigger drivers, 15s and up, they run bolts through to the rear because the wooden wings flex.
Most builders are using 0.5cm foamcore for smaller full range drivers but I wouldn't be told and used 1cm stuff instead. Glad I did, given how much bass this 5-inch driver can produce.
And I reinforced the front wings pretty well. Some double up the 0.5cm boards for the front panel.

Initially the bass was almost silly good, but I'd forgotten I had a bit of EQ in there. Removing 3.5dB made it much more even.
And I will modify the front panel tonight and resite it because this absolutely does reinforce the bass.

Foamcore is a pretty cool material for butchers like me who cannot build anything much better than Homer's spice rack.
So that's why I always go for simple wooden builds.

It doesn't help having a POS table saw either, but I've discovered I can do much better lines by tieing the fence down both ends with clamps.
Im going to need to extend the range of the tablesaw - there are four bolt holes to make it a bigger table, Im presuming they're for that- because my next subwoofer project involves a reasonably big box!

I had a large transmission line subwoofer complementing the Karlsonators and working in room down to about 20Hz.
It was driven by 2x10 inch Dayton subs back to back in a small box, out of which emerged a pipeline 3.5m long, snaking up and behind the TV and equipment stand and down the other side.
Anyhow, it sounded pretty good, at least the low notes did, but wasn't ideal, and visually the other half was forever on me to get rid of it.
So while working on a theoretical box TL which Im not yet ready to build, I managed to fashion a right sized box for the two 10s which sounds much better.
But once you've heard what TL subs can do...

Table-saw extension are good, but you need flat floor space & you rely on the fence anyway, so it may be easier to use a clamped straight-edge guide on a flat floor, with a circular saw.
Lift panels up with battens or a sacrificial foam or such.
These days, battery-powered saws are so much easier to use & inexpensive, esp for low-powered panel cuts.
I have a table-saw which is v handy, but I'm finding a cheap Ryobi cordless 165mm circular saw so easy to use (compared with my old Skilsaw).

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