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stevie

12" Cab Diary Continued

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Haha I said you'd end up being Gottastopbuilding gear :) I'm really pleased about what you say about the 3/4 ply with batten design, it won't be perfect but I found it was pretty good too.

I'll stick with the woodworking questions here, my posts tend to be too long.

All speaker design is compromise and I'm really pleased what you've said about my '18mm with battens' (actually reinforced butt joints) design. I chose these for two reasons, mainly using screws to draw the battens/panel joints together means you don't need any real extra clamping making it an easy build. I wanted everyone to feel this was something they could do at home with no real woodworking experience or special tools.

Secondly the battens double the glue area and double the strength of the cabs. I was aware however that 18mm panels are pretty rigid at these sizes and the battens do stiffen and damp the panels as well. A compromise which means you can get away with no extra bracing. That's not to say you couldn't improve things with bracing but you should get a good result without that level of complexity.

Lot's of other joints are available to you, dovetail, finger joints, dowels, biscuit joints and plain butt joints as Stevie used. Dovetail and finger joints double the glue area but won't be as strong as a reinforced joint and need to be clamped whilst the glue dries, crucially you need to have specialist tools to do the job. Biscuits and dowels are marginal for me putting a dowel into the sawn end of a 18mm panel isn't a strong thing to do and if you have to fit corners that means rounding off the panel weakening the joint even more. They may hold the joints square whist the glue sets but I still feel you'd need to clamp everything. Plain butt joints are just too weak for me, though Stevie's bracing will considerably stiffen and strengthen the cab I still think there is a good chance that the corner joints may break given any force applied at an angle, like dropping the cab for example. That is especially true of a cab made of 12mm ply.

Finally the reinforced butt joint is more forgiving of any mistakes in measurement or cutting, you are more likely to end up with an airtight cab.

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[quote name='Phil Starr' timestamp='1488298405' post='3247657']
Haha I said you'd end up being Gottastopbuilding gear :) I'm really pleased about what you say about the 3/4 ply with batten design, it won't be perfect but I found it was pretty good too.

I'll stick with the woodworking questions here, my posts tend to be too long.

All speaker design is compromise and I'm really pleased what you've said about my '18mm with battens' (actually reinforced butt joints) design. I chose these for two reasons, mainly using screws to draw the battens/panel joints together means you don't need any real extra clamping making it an easy build. I wanted everyone to feel this was something they could do at home with no real woodworking experience or special tools.

Secondly the battens double the glue area and double the strength of the cabs. I was aware however that 18mm panels are pretty rigid at these sizes and the battens do stiffen and damp the panels as well. A compromise which means you can get away with no extra bracing. That's not to say you couldn't improve things with bracing but you should get a good result without that level of complexity.

Lot's of other joints are available to you, dovetail, finger joints, dowels, biscuit joints and plain butt joints as Stevie used. Dovetail and finger joints double the glue area but won't be as strong as a reinforced joint and need to be clamped whilst the glue dries, crucially you need to have specialist tools to do the job. Biscuits and dowels are marginal for me putting a dowel into the sawn end of a 18mm panel isn't a strong thing to do and if you have to fit corners that means rounding off the panel weakening the joint even more. They may hold the joints square whist the glue sets but I still feel you'd need to clamp everything. Plain butt joints are just too weak for me, though Stevie's bracing will considerably stiffen and strengthen the cab I still think there is a good chance that the corner joints may break given any force applied at an angle, like dropping the cab for example. That is especially true of a cab made of 12mm ply.

Finally the reinforced butt joint is more forgiving of any mistakes in measurement or cutting, you are more likely to end up with an airtight cab.
[/quote]

No mention of pocket joints, which are my favorite for 3/4" plywood, and only take a minimal tooling investment. I sometimes use battens too, and one or two spine braces on larger panels. Woodworking is pretty much my least favorite aspect of DIY building though.

Edited by Passinwind

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Pocket joints and dowels combined could be quite good - a few dowels for alignment, and pocket joints to draw the pieces together. I have to say that I'm not hugely keen on the woodworking aspects myself. I had the ply cut to size for the cabs I made last year, and would do exactly the same again to save faffing around with squaring edges with a router (I don't have a circular saw).

Two more questions if I may. First, if someone wanted to maintain the same dimensions as the original build then could the port be put on the rear panel, and would there be any disadvantage in that? Second, does the added tweeter open up any alternative 12" drivers, since the need for a wide frequency range is reduced? I was looking at the Kappalite 3012LF recently, for example, though I don't know whether that would actually be any better - I've not got around to trying to model that in WinISD yet, and to be perfectly honest I don't think I know enough to be able to do that with any degree of confidence anyway!

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i don't want to steal Stevie's thunder but I will try you explain the thought processes after three beers.

Firstly, 15mm Poplar ply. The stock we can get is 9 ply and that is for either 15 or 18mm. To try to keep the weight down we have gone for 15mm. So 6mm dowels can be used with care. I have not tried pocket holes on 15mm stock so I cannot comment although I have a pocket hole jig and use it or other projects.

In theory there is no problem in porting at the back but again I will bow to Stevie's superior knowledge.

I know Stevie was planning on looking at alternative drivers/voicings to satisfy the Old School brigade but I don't think the Kappalites were what he was planning to use. The Kappalite 3012LF is a great LF driver but there is some debate as to whether it is ideal in a bass cabinet. I am going out on a limb here but the SM212 performs better overall than any of the Eminence drivers I have heard over the LF and mid range. Its problem, like all 12" speakers, is beaming. It is the beaming that we are trying to design out. To explain beaming, think of lighting. The two extremes are spot lights and flood lights. A spot sends out a narrow beam of light picking out a single performer. Great for Sinatra or Robbie Williams but... where's the band? Beaming is a spotlight and we want a floodlight.
The problem, with beaming for the working bassist is that unless you have ears in your knees, you need either a separate monitor, a second cab atop the first, IEMs, or a cab designed for wide dispersion to hear yourself on stage.

Having heard one of the Eminence drivers in the prototype MK2 and MK1 cab, the only thing the Kappalites will give you, i[b][i]n my opinion[/i][/b], is light weight and an empty wallet. The 3012LF has a peak in the response at about 2Khz. For the P-Audio tweeter the crossover frequency needs to be higher than that. The peak on the 3012HO is much higher but is not the LF beast that the 3012LF is. It has an Xmax of only(sic) 6.2mm compared to 8.25mm for the Beyma SM212 and 9.1mm for the Kappalite 3012LF. So the HO cannot move as much air. Of course the Kappalites are 2Kgs or 5lbs lighter and that may be a reason for using them.

[size=4]Both Eminence drivers have higher power ratings but the difference is small enough to ignore. The SM212 is rated at 350W, the 3012HO at 400 [/size]and the[size=4] LF at 450. The difference between 350W to 450W is under 1dB[/size] in an ideal world and 1dB is audio's JND (just noticeable difference) in ideal (quiet) conditions. Standing next to Animal you won't hear it. Of course the Kappalites have bigger voice coils (3" rather than the 2.5" for the SM212) so may handle more power that the raw specs would suggest.

If you want a flat response, HiFi voicing, then the SM212 is ideal. If you want Old School then other drivers need to be considered, Stevie has both the MK1 and MK2 prototype cabinets and they will stay with him so he can check different drivers. One last thing, don't rear port until ,Stevie has confirmed or trashed my views.

Edited by Chienmortbb

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Re-reading Phil's comments, I have to agree that there is a potential problem with plain butt joints on plywood. The adhesive works best on the face of the board and less well on the cut edges. Hence battening each edge solves that problem. The (partial) solution to this is to seal the cut edge with a 50/50 solution of water and PVA woodworking adhesive prior to making the butt joint. Of course we have not published the design as yet but the bracing is comprehensive and that can only add to the strength. The offline "discussion" as to how to construct the cabinet was "frank" and of course, Phil is right that the strength of a fully edge attend cab will be stronger that one without the edge bracing.

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Ha ha, you've all got me wincing at some of the woodworking suggestions, ouch!

Plywood is such a flimsy material, those plies are very thin and weak individually, and only held together with a fairly weak resin. Break away any glued panel and you inevitably come away with some of the outer ply attached, anything stuck at the cut edge is extra weak as that flimsy outer skin is only fixed on one side of the joint. Biscuits and dowel are made of compressed wood designed to expand inside their joints to make a tight fit running them into the plies further weakens them. Pocket joints are incredibly weak into thin panels as the screw tips barely pentrate the panel [url="https://woodgears.ca/joint_strength/pockethole.html"]https://woodgears.ca...pockethole.html[/url] In any case most cabinet corners need the edges of cabs rounded off to fit, you really don't want metal screw ends sticking into the place you are going to run a router bit. Honestly I suspect plain butt joints (just sticking the panels together with no extras) will be as strong as using dowels/biscuits or pocket joints, the weak spot is the outer skin on the cut edge.

What you have in your favour is that a box is an inherently strong structure. Stevie has rejected the reinforced butt joint for the perfectly good reason that it would make his bracing more difficult especially in a prototype. The bracing itself will itself stiffen the cab mechanically and increase the glue area. It's a good design, just a bit harder than the mk1 to make.

Putting in a 3012LF would work but the crossover is optimised for the drivers Stevie has chosen. I'd put the Eminence in a smaller cab probably, this cab is optimised for the SM212. I wouldn't put in a really pricey driver like that without optimising the cab for it.

Porting to the rear is fine, if you keep the volume of the cab and the dimensions of the port the same the cab will have the same resonant frequency and the same damping on the speaker. There will be some minor changes and you won't get exactly the measured results Stevie gets. The cab will be sensitive to being pushed right up hard against a rear wall, port resonances venting to the rear will be less noticeable and a number of cabinet resonances always change with even minor changes of the internal structure of the cab. None of these are likely to have a major effect upon the character of the cab

Edited by Phil Starr

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Honestly I think any of the jointing methods will be OK unless the cab is abused - I used biscuit joints on my build primarily because it makes it far easier to align the panels vs plain butt joints (although the cleats to reinforce the joint Phil suggests would also be a simple way to do that without any special tools).

I agree dowels/biscuits on a lot of low-quality ply is a bad idea (the core is often full of voids and very weak), but if you make the effort to find good quality void-free birch ply it'll probably work fine with any of the methods mentioned, particularly when you consider the strength added by securely attaching (ideally glueing) the back of the cab and the baffle.

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If you have the skills, use whatever fancy jointing method you like. The rest of us will use normal butt joints, which are perfectly fine and used by everyone and his dog for this kind of cabinet. Remember, we are not building furniture. Here is a video showing how to screw a box together. It's not difficult. Fast forward to about 3 minutes 20. https://www.youtube.com/watch?v=L5tkfhXjSTk

Here's another guide on how to assemble a speaker cabinet: http://www.speakerplans.com/index.php?id=guide

Once the glue has dried, remove any screws close to where you intend to round over and fill the holes with car body filler (not wood filler). You can leave the other screws where they are and just fill over them.

Marco, also known as Ghost_Bass, has been working on the cab drawings. When they are up, they should answer a number of the questions that have been raised.

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Chienmort and I will both be building one of these cabs from poplar ply in a few weeks' time and we'll be able to provide a more definitive answer on weight, as well as providing a step-by-step build guide. As the cabinet volume of the MkII box is identical to the MkI, the weight should be the same, but calculate 1kg for the tweeter.

Putting the port on the back is fine. Position it 2/3 up in between the braces. You will probably have to space the braces out a bit more, but that's OK.

The Eminence 2012LF is a pure woofer and needs to be crossed over no higher than 1kHz to a midrange unit. Alternative bass drivers are possible but would mean a crossover redesign. Now that Bergantino has discontinued its CN112, an interesting lightweight possibility would be the neodymium Faital Pro PR300 with the neo version of the P. Audio tweeter.

Edited by stevie

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Bracing is vital to the performance of this box - it's not an afterthought. I have the MkI and the MkII cabs in my workshop, and I can say without a shadow of a doubt that there is no comparison between them as far as panel stiffness is concerned. The 1 to 1.5kg that went into reinforcing the butt joints in the MkI is now doing a much more useful job stiffening the whole of the MkII cabinet.

Siegfried Linkwitz has this to say on the subject.

"Conventional box loudspeakers very often suffer from spurious sound radiation, which is caused by the mechanical vibration energy of the drivers being transmitted into the cabinet and exciting the cabinet walls to vibrate at certain panel resonance modes. Furthermore, the high sound pressure levels inside the cabinet can excite panel modes. Since the cabinet's radiating surface areas are usually much larger than the driver cone area [i]even relatively small panel excursions can lead to significant spurious acoustic output.[/i] Depending upon the cabinet construction [i]the output might even be larger at certain frequencies than the desired output from the driver[/i].

Knocking with your knuckle on a panel can give you a rough idea of the dominant mode frequency, though it might not necessarily get excited by the driver. This test can also tell you how stiff the panel is, when the pitch is high, or how well the panel is damped because it hurts to make it respond.

There are several ways to reduce modal panel vibrations. Because the vibration energy from the driver decreases rapidly with increasing frequency it is advantageous to push the panel vibration modes up in frequency where the excitation energy is small. [i]This is best accomplished by increasing the panel stiffness[/i], but often goes together with increasing the mechanical Q of resonance. As my rule of thumb, no un-braced box panel area should be larger than 4 inch squared for 3/4 inch thick wood panels. That is a lot of bracing, but it pushes modes into the low kHz range."

Just to clarify - for those that get a headache reading technical stuff - when you play your bass through a conventional wooden cabinet, the cabinet itself can produce more output than the driver on certain notes. Because the radiating area of the cabinet is much larger than the radiating area of the driver, the cabinet doesn't have to move very much to do this.

The bracing is not there to stop the cabinet rattling; it's there to stiffen the cabinet and stop it "playing along" with the driver. You only realise how much mush the cabinet contributes to your overall sound when you compare it with a cabinet that has been properly treated. A cabinet designed for acoustic performance (rather than ease of manufacture) is demonstrably cleaner, tighter, more natural and has more punch.

Edited by stevie

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[quote name='stevie' timestamp='1488469341' post='3249232']
Bracing is vital to the performance of this box - it's not an afterthought. I have the MkI and the MkII cabs in my workshop, and I can say without a shadow of a doubt that there is no comparison between them as far as panel stiffness is concerned. The 1 to 1.5kg that went into reinforcing the butt joints in the MkI is now doing a much more useful job stiffening the whole of the MkII cabinet.

[/quote]

So if someone was to build a Mk1 cab now (not wishing the extra complexity of a tweeter and crossover) would it be best to use a similar bracing pattern to the Mk2? Or does the Mk1 cab stand as is?

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If you want to build the MkII cab without the tweeter, just leave the tweeter out. You then have the option of adding the tweeter any time you like. The tweeter and crossover will only cost about £25 total, but they do represent a useful upgrade in sound quality. I'm going to do a crossover layout so that you can see how to assemble it and Ghost_Bass has kindly offered to do some proper drawings of it.

Edited by stevie

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I would recommend building the MK2 but it is more complex You could put the new port into the MK1 but the baffle is smaller and so there will be np room for upgrading later. I suggest you what until Ghost-bass and Stevie have finalised the drawings and then decide. As far as I know they ar a few days away. I will do a MK1 v Mk2 comparison later if it helps.

Edited by Chienmortbb

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Here are the drawings. A very big thank-you to Marco (Ghost_Bass) for doing these.





I’m attaching the higher resolution PDF to Post no. 1 in this thread (when I figure out how to do it) and will update that with any future amendments.
Please note that the braces G, I, H and J do not have to be cut out of a sheet. They can just as easily be made from 12 or 15mm x 32mm batten.

Edited by stevie

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[quote name='Chienmortbb' timestamp='1488534306' post='3249690']
I would recommend building the MK2 but it is more complex You could put the new port into the MK1 but the baffle is smaller and so there will be np room for upgrading later. I suggest you what until Ghost-bass and Stevie have finalised the drawings and then decide. As far as I know they ar a few days away. I will do a MK1 v Mk2 comparison later if it helps.
[/quote]

If you look take a closer look at the drawings, this box isn't really that hard to build because the baffle, back and all the braces are the same width. That should help to line everything up neatly. The only difficult bit is cutting the 5" hole for the port. If anyone would like to borrow my 5" hole cutter (and has a drill and an arbor) they are quite welcome. Just post it back to me when you've finished, or perhaps post it on the the next builder.

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That's a pleasingly clear set of drawings, and it looks like you should be able to get a pair of them out of a standard sized sheet of ply too. Is there a typo in the side panel size though? It looks like it says 510mm while the baffle and back are 540mm, or is it a combination of the image resolution and me not having my glasses on?

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Yes, the idea was to get two out of one sheet of plywood. John (Chienmortbb) is going to produce a cutting list next week that anyone should be able to take along to their local wood supplier to get the panels cut to size.

You're right about the 540mm. Sorry about that. I'll check all the dimensions against the prototype cab this afternoon - that's the beauty of building a prototype :) . We'll fix any discrepancies in the PDF - and if anyone else sees anything that's not quite right, please say.

Edited by stevie

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Nice diagram and detailed design, great work folks, thanks!

I think I'm going to have to build one of these to see how it compares to my current SM212 loaded cabinet :)

My SM212 is from early 2013 - was there a spec change for newer drivers?

I seem to recall something about that, so wondering if I'm better getting a new one for this build, or can I safely switch the existing driver between cabinets?

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Yes, you can used your existing drivers. Beyma just changed the way they presented the specification. I measured the T/S parameters of Phil's original driver, which is probably older than yours, and Chienmortbb's driver, which is quite recent. The parameters were comparable - so I'm quite sure the driver hasn't changed at any time during its production.

Edited by stevie

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What I don't know, I have to say, is whether the spec of the P. Audio horn has changed over the years, as mine is a few years old.

To save me buying another one, it would be good if someone could send me theirs for a couple of days so that I can check.

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[quote name='Chienmortbb' timestamp='1488534306' post='3249690']
I would recommend building the MK2 but it is more complex You could put the new port into the MK1 but the baffle is smaller and so there will be np room for upgrading later. I suggest you what until Ghost-bass and Stevie have finalised the drawings and then decide. As far as I know they ar a few days away. I will do a MK1 v Mk2 comparison later if it helps.
[/quote]

A Mk1 vs Mk2 comparison would be very helpful thanks.
There are a few reasons why I'd be favouring the Mk1 build to the Mk2. Cost and complexity of the tweeter and crossover, the port design being a bit more involved, and the slightly more 'hi-fi' nature of the Mk2. A concern would be that the Mk2 without the crossover and tweeter would not perform as well as the Mk1 due to the different drivers. I just wondered if the Mk2 bracing would be an upgrade to the Mk1 cab?
From what I've read of those who have built a Mk1 it sounds ideal for me. The Mk2 might be extra everything when I don't need it IYSWIM. ;)

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[quote name='fftc' timestamp='1488547049' post='3249892']
A Mk1 vs Mk2 comparison would be very helpful thanks.
There are a few reasons why I'd be favouring the Mk1 build to the Mk2. Cost and complexity of the tweeter and crossover, the port design being a bit more involved, and the slightly more 'hi-fi' nature of the Mk2. A concern would be that the Mk2 without the crossover and tweeter would not perform as well as the Mk1 due to the different drivers. I just wondered if the Mk2 bracing would be an upgrade to the Mk1 cab?
From what I've read of those who have built a Mk1 it sounds ideal for me. The Mk2 might be extra everything when I don't need it IYSWIM. ;)
[/quote]The MK2 was never meant to replace the MK1. It was meant to be another offering indeed my build with the Compression Horn will be a Mk2+ but will be a lot more expensive than the MK2 as presented here.

The real advantage of the MK2 is the single large port but that also makes it more difficult to make. The MK1 was designed so that anyone could build it. However with Stevie's kind offer of the loan of a hole cutter it will be much easier to make the MK2.

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Agreed. I think it's fair to say that the goal of the MkI was to provide a cab that was compact, easy to build, produced good bass and didn't cost a fortune. And I believe it succeeded in doing that. The MkII is really a completely different design - the only common factor being the bass driver and the cabinet volume. This time, the goal is to offer a DIY project at a relatively low cost that performs as well as or better than most commercial offerings out there, and offers the satisfaction - which to some of us is priceless - of having "rolled your own".

By the way, fftc, both cabs use the same bass driver.

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That's right, the MK1 was designed so that anyone could build it with no special skills or tools. and if you prefer a tweeterless cab it's a pretty decent one.

one of the reasons we've designed these cabs so publicly is that you can adapt them if you want, and I hope share any experiences you gain. I recommended 18mm ply in the MK1 to avoid any complex bracing. It'll match most commercial cabs. Stevie and John wanted to push the envelope further and I cant wait to hear the results.

There's no reason why you shouldn't build a mk1 and add in more bracing at a later date or a mk2 without a horn and crossover. Between us we can guide you through the process if you want. Our intention is to demystify speaker building and what is going on inside your commercial unit if you don't.

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[quote name='Phil Starr' timestamp='1488564251' post='3250127']
That's right, the MK1 was designed so that anyone could build it with no special skills or tools. and if you prefer a tweeterless cab it's a pretty decent one.

one of the reasons we've designed these cabs so publicly is that you can adapt them if you want, and I hope share any experiences you gain. I recommended 18mm ply in the MK1 to avoid any complex bracing. It'll match most commercial cabs. Stevie and John wanted to push the envelope further and I cant wait to hear the results.

There's no reason why you shouldn't build a mk1 and add in more bracing at a later date or a mk2 without a horn and crossover. Between us we can guide you through the process if you want. Our intention is to demystify speaker building and what is going on inside your commercial unit if you don't.
[/quote]You and Stevie always seem to put it better than me, but that is exactly as I see it. Either cab is OK as they both have the same volume, just different dimensions. The Mk2 is easier to mod as we had different design goals.

I would happily gig with the MK1.

Edited by Chienmortbb

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