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Phil Starr

1x12" Cab Design Diary

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[quote name='redstriper' timestamp='1398349853' post='2433268']
Looking good Phil and well done for keeping the weight down.
Do you have an Eminence kappalite 3012LF driver to try in there?
I am particularly interested in it's low end capacity for dub reggae.

Steve.
[/quote]

I know weight of gear has been a hot topic with bassists for some time. My personal view is that a cabinet of this size should be made out of 18mm birch because it will be substantially more rigid yet will only to add 3.5kg to the total weight (even less when you consider the extra bracing needed for a 12mm ply cab). Everybody's needs are different though, and light weight remains a 'must' for many. Phil's choice of the Beyma is quite interesting, as it is relatively lightweight for a ceramic driver of this quality.

I have a couple of 3012LFs, one of which will no doubt find its way into one of these boxes eventually. I know it probably doesn't bother you too much, Steve, but that chassis really needs a midrange unit to function properly (as Phil suggested). Now that we finally have a box, Phil has asked me to post some simulations of some of the more popular drivers on the market, including some we do actually physically possess, and I'll try to do that before the weekend. I think these might surprise a few of you......

Edited by stevie

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Good... proper ports and not those horrible shelve things..
and 18mm Ply and the additional 3.5kgs weight wouldn't worry me either.

Just sayin'

Edited by JTUK

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[quote name='JTUK' timestamp='1398434635' post='2434098']
Good... proper ports and not those horrible shelve things..
[/quote]

Haha! Both are equally valid design concepts, one is not more "proper" than the other ;) Oh and Phil says further up that the final design will probably use the shelf type ;)

:P

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You can make quite a good case for the superiority of round ports, but I know Phil is using them in the prototype because they are easier to swap out.

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Here are the simulations of the first three 12" drivers in our prototype box. I've tuned the cabinet to 50Hz in all cases. I'll post some more later, but I'm restricting this to three at a time for the sake of clarity.

[IMG]http://i60.tinypic.com/2r4oow8.jpg[/IMG]

Yellow is the Beyma SM212
Green is the Eminence Beta 12
Pink is the Celestion BL 12-200X

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And here are another two popular 12" drivers in our box. All ceramic magnets so far.



Blue is the Fane Sovereign 12-300
White is the Eminence Delta 12LF

Edited by stevie

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[quote name='stevie' timestamp='1398439237' post='2434149']
You can make quite a good case for the superiority of round ports, but I know Phil is using them in the prototype because they are easier to swap out.
[/quote]

Absolutely, but slot ports are still "proper" ;)

Anyhoo just being facetious really, it's a great thread and v interesting :)

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[quote name='JTUK' timestamp='1398434635' post='2434098']
Good... proper ports and not those horrible shelve things..
and 18mm Ply and the additional 3.5kgs weight wouldn't worry me either.

Just sayin'
[/quote]

the whole beauty of a project like this is that you are building it and you are in charge. If you want round ports not a shelf then it's an easy substitution, in all probability we will use the same port area so the lengths will be the same. My intention for this design is to make it as easy as possible to build so people will be encouraged to have a go and will then almost certainly be successful. The big advantage of a shelf port is that you can get B&Q to cut everything and all you need to do then is stick it together. We'll give enough details to do round ports too. My cabs will almost certainly be 3/4 ply. we'll also give enough details to adapt cabs for other drivers and to design your own kickback.

Edited by Phil Starr

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OK, here is a quartet of neo drivers. The Faital is used in the Bergantino CN series as far as I am aware, and the others are also found in commercial cabs.

[IMG]http://i61.tinypic.com/j7usyv.jpg[/IMG]

Pink is the Eminence Kappalite 3012LF
Red is the Eminence Kappalite 3012HO
Yellow is the Faital PR300
Green is the Celestion BN120300X

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[quote name='stevie' timestamp='1398439467' post='2434154']




[/quote]
[quote name='stevie' timestamp='1398440762' post='2434173']




[/quote] Stevie - what causes the wobble just below 500Hz? I'm presuming something in the cab design as it's common to all the drivers

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Luke, it's the natural resonance of the port. I specified 4 x 70mm ports, as that's what Phil has fitted to the prototype. Unfortunately, that requires a port of around 10/11 inches in length which gives a low resonant frequency. It would be problematic leaving it like that because it would be audible, but there are ways of fixing it (in the mix!).

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Just to add to my previous post, there are a number of ways of minimising the pipe mode resonances produced by ports. The first one is to shorten the port, which increases the frequency and reduces the amplitude of the resonance. If you halve the port length you double the frequency of the pipe mode resonance, which will generally put it out of harm’s way on a bass guitar cab. The downside is that you have to reduce the diameter of the port at the same time, which may reduce the power handling of the port more than you would like.

This is just one of the balancing acts you are faced with when designing a speaker cab. The problem is more difficult with smaller cabs.

If you don’t want to, or can’t, make the port any smaller, a good solution is to put the port on the back of the cabinet, preferably near the centre of the cab. If you don’t want to do that, you can put a notch filter in the crossover to take out the offending frequency (you need to be a bit anal to do this, but I've seen it done).

The final solution would be to use a midrange driver that covers the frequency where the port resonance would be.

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[quote name='Phil Starr' timestamp='1398446897' post='2434290']
the whole beauty of a project like this is that you are building it and you are in charge. If you want round ports not a shelf then it's an easy substitution, in all probability we will use the same port area so the lengths will be the same. My intention for this design is to make it as easy as possible to build so people will be encouraged to have a go and will then almost certainly be successful. The big advantage of a shelf port is that you can get B&Q to cut everything and all you need to do then is stick it together. We'll give enough details to do round ports too. My cabs will almost certainly be 3/4 ply. we'll also give enough details to adapt cabs for other drivers and to design your own kickback.
[/quote]

Sure...I can see the ease of build and to make easier adjustments to tune whilst you settle on a final
design, making perfect sense.

My wishlist would include 18mm ply and round front ports in an ideal world, and weight-in at around 40 odd lbs
But then I'd also get a decent cabinet maker to put it together for me so the shelf port would not be a feature just because
it is easy to make...from my point of view.
But as a thread and a design exercise, this is a good read.

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[quote name='stevie' timestamp='1398357869' post='2433383']
I know weight of gear has been a hot topic with bassists for some time. My personal view is that a cabinet of this size should be made out of 18mm birch because it will be substantially more rigid yet will only to add 3.5kg to the total weight (even less when you consider the extra bracing needed for a 12mm ply cab). Everybody's needs are different though, and light weight remains a 'must' for many. Phil's choice of the Beyma is quite interesting, as it is relatively lightweight for a ceramic driver of this quality.
[/quote]

Another alternative is to take the compromise route and use 15mm ply - I did this with my (Beyma loaded) 1x12 build and it's worked out very well - only needed minimal bracing and it's very rigid (I think the shelf port helped here).

I've not weighed it yet, but (although it has two handles) it can be easily lifted with one hand, and it's simple to carry the cab with gig-bag containing amp on my back for one trip from the car at gigs.

Disadvantage is 15mm ply is harder to source, but a good timber merchant should be able to get it even if it's not standard stock, after a few calls I found one local-ish which had 15mm birch on the shelf and could cut everything to size for me.

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Any plans to experiment with thinner panels and tension bracing them?

I was kinda suprised that there were no braces in evidence on the test cabs(that i could see from the pics anyway), but if you can get acceptable results with 12mm and no bracing, are you not interested to find out if you can push the envelope and make a thinner panel work?

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The point with unbraced 12mm is to get unacceptable results and see what needs to be done to get them acceptable, if that makes sense. Tension bracing is a flawed concept with wood because of viscoelastic creep - it won't keep the tension over extended time periods

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for the stupid amongst us.... what's tension bracing?

I've seen a few types of cab bracing around. I don't know the proper names!
Fearful seem to favour using wood on the panels to stiffen it - a bit like a acoustic guitar top.
Bill Fitzmaurice seems to advise a grid of dowels pushing out, I think Andy at ACME does similar.
And Barefaced seem to (possibly) use some simple matrix type bracing.

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I deliberately built the prototype out of thin panel to investigate the panel resonances and then to investigate optimum bracing. the result is truly awesome at the moment. The cabs sound great at low volumes and dreadful once the volume gets high enough to excite the panels, The final versions will probably be 3/4 and I'll then do some comparisons between 1/2" extensively braced and 3/4 with little bracing.

Remember this is just a test bed.

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I'm assuming that what's referred to as tension bracing means just that, panels are placed under tension which has the effect of greatly increasing stiffness, like a strung bow. But wood gradually loses tension like this over time (hence why bows are stored unstrung).

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[quote name='LukeFRC' timestamp='1398614305' post='2435733']
for the stupid amongst us.... what's tension bracing?

I've seen a few types of cab bracing around. I don't know the proper names!
Fearful seem to favour using wood on the panels to stiffen it - a bit like a acoustic guitar top.
Bill Fitzmaurice seems to advise a grid of dowels pushing out, I think Andy at ACME does similar.
And Barefaced seem to (possibly) use some simple matrix type bracing.
[/quote]
I was referring to the BFM/ACME ram in a bit of wood that is a little longer than the inside dimension of the cab method - no idea if it is actually correct terminology, but it made sense to me. It looks a little crude at first, but makes a lot of sense as all materials become stiffer as you shorten their vibrating length.

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Making it a little longer will initially put the wood under tension, which will act to further stiffen the panel, but after a while the tension element will be lost so it has no benefit over a brace that's exactly the right length - unless the brace is long enough to put a significant curve into the panel in which case there is a geometric stiffening effect (like an arch) but this is separate to the tension/stress element.
Regarding brace orientation, from previous posts BFM and I think Phil favour the dowel approach but I'm not convinced it's sufficient on a thin-walled full-range cabinet because there will be multiple vibrational modes where the brace sits at a null, or where opposing panel attachment points are vibrating in phase, cases where sufficiently stiff axial braces ought to be more effective. But it'll be interesting to see in practice what effect different schemes have, I suspect outcome is going to be very material- and application-dependent.

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