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stevie

12" Cab Diary Continued

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One of the things about this process is that it is a design project. As with the original design process, where the slot port caused a couple of problems, I expect other issues will be thrown up that make us question the way speaker cabinets are often built.

Going back to the start of the thread. Try Stevie's finger test on your own cabinets and see if any of the panels resonate at particular frequencies or notes. Also let us have the results on here by naming and shaming.

Edited by Chienmortbb

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I haven't quite finished the bracing, but the cabinet walls are already impressively quiet. I've been using lengths of 12mm x 32mm softwood (as mentioned by JPH) , which are readily available from local wood merchants and the usual DIY stores. The advantage of using these is that they are cut perfectly square, which makes it easy to get a tight glued connection between the brace and the cabinet panels. The secret of getting the brace clamped tightly to the panel is to use wood screws and to pre-drill through the brace into the panel (not right through it, obviously). Use plenty of PVA and make sure it oozes out from both sides right along the length of the brace.

5x245z.jpg&key=ee065a8c23b23d935d6204afd9a020128b07a177e55b3c75c6b60c9ffd4e9e48

fp4h3o.jpg&key=009d357d08dacc92f1e331156c829c841f4259b2b1d11ac86bd32055cbed9a18

The most important brace is a figure-8 brace just above the driver, as this cuts down vibration on the baffle and stiffens the cab close to its centre point. There is another semi-figure-8 below directly behind the driver with sufficient space to allow for the driver and magnet.

With a birch ply box, that is probably all you would need. The remaining two braces are glued to the back and side walls in a "U" shape. One set positioned near the top of the cabinet, and one set near the bottom. The figure-8 configuration cross brace is not needed here because the top and bottom panels provide sufficient stiffness. I ended up with four circular or semi-circular braces fairly evenly spaced along the length of the cabinet.

By the way, this bracing arrangement was arrived at systematically using the sine wave test tones from my Alan Parsons test CD. I still have to brace the bottom and top panels. Then I’ll report on the final results.

[PLEASE NOTE: THIS WAS A PROTOTYPE. THE FINAL BRACING DESIGN APPEAR LATER IN THE THREAD.]

Edited by stevie

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I realise the compromises we took using Spruce, it was what I had in the garage. However I am surprised how much bracing is required. Birch is not twice as strong as Spruce or Poplar and going down to 12mm or even 9mm in some cases, I wonder how well commercial cabinets are braced*. The other problem you have is less surface area to glue to, especially when the panel edge, does not take adhesive as well as the face of the panel.

I know Phil prefers to use battens on all panel joins for the extra strength. I wonder how much extra bracing would be needed with that approach? Also would plywood offcuts, the same size as the softwood, have the same effect?

*The poplar we plan to use for the final design, although 15mm, is 9 ply so will be stronger that the spruce (18mm 6 ply).

Edited by Chienmortbb

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Two things primarily affect panels of the same dimensions, mass and Young's modulus (the bendiness) you'd be better off sonically with heavy MDF which is also self damping to some extent. Obviously if you start off with the idea of light weight then heavy panels aren't an option. That means extra bracing.

The main reason that I recommend battening all the panel joints is for ease of construction and increased strength of the final cabinet. The screws draw the joints together whilst the glue sets, the battens hold everything square and they also double the glue area. There will be a little bracing effect and some damping of panel resonances but that is incidental if I'm honest.

As you observed John, all speaker design is a bit like squeezing a balloon, an advantage gained somewhere almost always leads to a cost elsewhere. That's what makes it interesting :)

Edited by Phil Starr

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I found that the best way to brace the end panels was across the longest dimension. When the glue dries the bracing will be finished.

2dv0mev.jpg

I needed two lengths of 2.4m batten to effectively brace this particular cab. I forgot to weigh the batten before I started cutting it up, but I calculate that it adds around 1.2kg to the overall weight of the cabinet. Using poplar ply strips or shelves might reduce this a little.

I’ve carried out this exercise on plenty of cabinets before. It is a particularly good way of “fixing” an inexpensive shop-bought cabinet, as these are hardly ever braced adequately.

Edited by stevie

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I've used both softwood and ply to brace my builds, but always rebated into the panel. My 'test' was far less scientific than Stevie with his test tones, but I found that whilst ply 'on edge' added a lot of rigidity to the panel, the softwood wasn't far behind and it definitely had a damping effect which i guess is because the softwood would have a different natural frequency to the ply.

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Yes, I would also expect plywood to be stiffer than the softwood I’m using, JPJ. The softwood is actually quite stiff on edge, but the big advantage for me is that I can pick it up from the local wood merchant and simply cut it to length.

I’ve now carried out sine wave testing on the fully braced cab and this proved quite interesting. Without boring everyone with all the figures, I found that the spurious vibrations at the fundamental frequencies of the bass (40 to 100Hz) had virtually disappeared and that the level of vibration at all other frequencies was much lower than before. This will have a positive effect on the sound of the system. It will also ensure that amplifiers placed on top of the cab will not dance about or fall off – perish the thought!

Now that the cabinet is so much quieter, the sine wave tests showed that some more work needs to be done on the porting. Test tones are very good at revealing problems that you’d miss just using a bass as a sound source. So the next job is to take a closer look at the porting arrangement to make sure it’s optimum.

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[quote name='JPJ' timestamp='1486160392' post='3229824']
I've used both softwood and ply to brace my builds, but always rebated into the panel. My 'test' was far less scientific than Stevie with his test tones, but I found that whilst ply 'on edge' added a lot of rigidity to the panel, the softwood wasn't far behind and it definitely had a damping effect which i guess is because the softwood would have a different natural frequency to the ply.
[/quote]
I've braced my cabs with plywood shelves, but used a foaming polyurethane glue, as recommended by Bill Fitzmaurice to ensure airtight joints - is that likely to provide more damping than a solid 'resin W' type glue joint?

David

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A bit too much chuffing for my liking, John. Sorting the porting out has taken a while and lots of hole cutting and gluing with glass fibre and filler. However, I do believe I now have the solution, which I will post later today.

David, PU glue is great when you don't have accurate joints and need to fill gaps, but these braces really do need to be pressed very tightly to the cabinet panels for optimal effect. If there is a slight gap because the brace (or shelf) hasn't been cut correctly or fixed down tightly, it won't work properly. While the gap-filling properties of polyurethene will prevent vibration if the joint isn't perfect, you really don't want any gaps at all.

The damping effect of attaching bracing comes more from stiffening the panels than any properties of the glue. In fact, you can hear the effect of the brace just by pressing it tightly on the panel without gluing - which is how I arrived at the positioning of the bracing for this box.

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[quote name='stevie' timestamp='1486459294' post='3231866']
...David, PU glue is great when you don't have accurate joints and need to fill gaps, but these braces really do need to be pressed very tightly to the cabinet panels for optimal effect. If there is a slight gap because the brace (or shelf) hasn't been cut correctly or fixed down tightly, it won't work properly. While the gap-filling properties of polyurethene will prevent vibration if the joint isn't perfect, you really don't want any gaps at all.

The damping effect of attaching bracing comes more from stiffening the panels than any properties of the glue. In fact, you can hear the effect of the brace just by pressing it tightly on the panel without gluing - which is how I arrived at the positioning of the bracing for this box.
[/quote]
OK thanks for that.

David

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This is the port that eventually worked best and which generates very little port noise even at fairly high levels.

[url="https://postimg.org/image/km8htctt5/"][/url]

It’s a 120mm Manrose pipe (125mm external), available from Screwfix (and lots of DIY and plumbing stores). You can cut two ports from it and it costs £2.69.

I tried various sizes and types of commercial port, but this was definitely the one. In the process, I cut and filled so many holes, my baffle ended up looking look like this:

[url="https://postimg.org/image/x8756ba9r/"][/url]

This is the hole cutter I used, which produced a really snug fit in one go.

[url="https://postimg.org/image/3wg83i137/"][/url]

If you don’t have one (which is admittedly very likely), you will need to use some kind of hole cutter or router to cut the hole. Unfortunately, there is no commercially available port in this size with a lip to hide dodgy woodwork.

I am now making a start on the internal damping.

[url="https://postimg.org/image/6q42ragcd/"][/url]

Edited by stevie

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I wonder if anyone has made a nice tapered port with a flange (for hiding sub-par hole cutting) using 3-d printing? I've read that tapered (not sure that is quite the right expression)port exit can give lower chuffing for a given original diameter.

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Yes, one big port, Luke. The idea is to minimise friction and turbulence within the port; the less friction, the less chuffing and compression. It's not so critical with large cabs, or general purpose PA cabs for voice, but with small, high powered bass cabs like this one, optimising the porting makes a *huge* difference to the output produced by the port - and reduces distortion audibly. A single round port is simply the best solution for minimising friction.

They're called flared ports, MoonBass. I have a selection of them here, including some very nice Precision Ports 4" ones. Unfortunately, availability is a bit of a problem in the UK, but they do work really well. I considered using the Precisions, but their external diameter is about 18cm and there wasn't enough room on the baffle. Also, a single flared 4" port wasn't going to be long enough to tune the cab to the target frequency. The performance of the 5" we are using is equivalent to a flared 4" port. The ideal would be a 5" flared port on the back of the cab, but nobody makes one.

If somebody could use a 3D printer to make some flared port ends cheaply, that would make me very happy. It seems like an ideal job for a 3D printer.

Edited by stevie

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[quote name='LukeFRC' timestamp='1486507747' post='3232428']
So just the one big port?
[/quote]
I think it's a good decision

Stevie found it was relatively easy to create chuffing in the ports with his test signal and multiple small ports. Since his main aim was to get the best out of this cab it made sense to address that. The aim for the first design was around making it easy to build and the ready availability of black downpipe and hole cutters that size drove a decision on multiple ports for my design. Our aim in sharing this stuff is to give people a design they can follow exactly if they want, with a guaranteed performance, but also to give enough explanation and data for anyone to modify the designs easily enough if they choose.

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[quote name='stevie' timestamp='1486548855' post='3232603']

If somebody could use a 3D printer to make some flared port ends cheaply, that would make me very happy. It seems like an ideal job for a 3D printer.
[/quote]
Well, that's my good idea for the year, and it's only February :(

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Hey, you could print a curved tube port for a small cab where a normal tube port wouldn't fit. Grooveh!
2 ideas in one evening?
I'd better go to bed now before my foot falls off!

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This is what the inside of the cabinet looks like now.

[url="https://postimg.org/image/t91t3ld7t/"][/url]


I’ve been using felt on a lot of my boxes recently (a wool/cotton mix) and I’ve found it works exceptionally well. I did try foam and BAF wadding in the cab, but they weren’t as effective. As usual, the material is inexpensive and readily available from eBay. £6.99 (delivered) gets you a square metre, which is enough for two cabs. I’ll list information on where to get all of the bits and pieces when we're finished.

Use one layer all round except for near the port, with two layers on the bottom 25% of the back panel. This arrangement was arrived at by systematic measurement and is not just a best guess. The felt needs to be glued to the cabinet panels, with a few staples to keep the second layer in place. Contact adhesive is best (just apply to one side). I found this spray can of contact adhesive in Poundland, which makes the job easier. It contains enough for one cab:



Not much more to say about this, except that if you are thinking of putting damping into your current cabinet, this is a good way of doing it.

Now that we have a sorted cabinet, we can start designing the crossover. I’ve put a handle on the side and this is what the cab looks like with the drivers in:



If you’d prefer a black port, it can easily be painted black with something like Holts bumper paint.

The two raw drivers we'll be working with measure something like this:

[url="https://postimg.org/image/pik463nxt/"][/url]

Edited by stevie

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Here is the crossover circuit (it's not very clear, but the first inductor is .6mH and not 6mH):

[url="https://postimg.org/image/nfy4sbfz1/"][/url]
Film caps throughout. 1mm wire on LF coil, .7mm on HF coil. 17W resistors on HF. 30W minimum on LF (2 x 17W/15 ohm resistors would be OK). If you need a layout, I can do one, but it's quite a simple circuit.


Here are also a few measurements I took of the finished system. They are anechoic measurements without any smoothing. The curves that look a bit rougher than others were taken further away and are not 100% anechoic, as they contain some reflections. Ignore the droop to the far left of some of the measurements – it’s an artefact.




Showing the tweeter response. Crossover point is 3kHz.

[url="https://postimg.org/image/yp4m6xw9b/"][/url]


Impedance. This is a very easy amplifier load.

[url="https://postimg.org/image/4c8vjwbyp/"][/url][url="https://postimage.org/"]imgurl[/url]


Response 30 degrees off axis (top curve is on axis). You can see that the frequency response collapses between 2 and 3kHz because the bass driver is beaming too much here.

[url="https://postimg.org/image/7ezc5xd6b/"][/url][url="https://postimage.org/"]uploadimage[/url]



This is what you can expect if you are of average/above average height standing 1 metre in front of the cab (upper curve is directly in front of the cab). The response of any 12” driver starts to collapse at 1kHz off axis, and you can see that happening here. However, the output from the tweeter helps compensate for this drop from around 2kHz, which will help the player to hear what he is playing.

[url="https://postimg.org/image/x963q3yk7/"][/url][url="https://postimage.org/"]upload pics[/url]

I would stress that this is not your usual boom/tizz tweeter solution. Here, the tweeter really helps to define the sound of the bass. Once you have played a system like this, you are unlikely to go back to a single driver design.

I’m trying to get drawings sorted now so that anyone can build one of these. I’ll post as soon as they are available - hoping for end of the week.

I have also finished the design of a 12 + compression driver and horn system that Chienmort intends to build. It uses exactly the same cab and bass driver but has a Celestion compression driver and a CD horn, which will obviously cost a bit more than this tweetered version. The crossover is a lot more complex because of the CD horn and lower crossover point. If there is any interest, I'll post details of that here too.

Finally, it would be really helpful if anyone who owns a top-end 12” bass cab and is not too far from Dorchester could arrange to pop in to see me for half an hour so that we can compare that cab with the Basschat 12 I have here, and also take some measurements – whether for publication or not. Just message me and we’ll get something sorted.

Edited by stevie

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As Stevie says I am going to build the cab with a Celestion Compression Driver coupled with a P-Audio horn. The horn has unusually wide dispersion and should be useful for allowing me to hear myself even when just in front of the cabinet.

We really need some commmercial cabs for comparison purposes. I have a TC BC212, it is a mid range cab but it would be really helpful to have more cabs that we can test in controlled conditions. Can you help?

The knowledge we are gaining and putting on here is useful even if you don't want to build a cabinet. Some of the findings have surprised me and with open minds, free of commercial pressures, we have no axes to grind. The port situation is a good example. One big horn (ooh err Mrs) is a lot better than a few small ones. It is likely that multiple slot ports are also an issue for exactly the same reason, who knows what else we will find with enough cabs to check.

Anyway thanks for reading and if you could help in any way....let Stevie know.

Edited by Chienmortbb

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As the proud builder and owner of two of the original Basschat 1x12's (and as someone who's just sold a 2x10 cab due to lack of storage space) I shouldn't really be interested in this. But... I am, so I'll chime in with a few comments and questions.

I used 18mm Poplar ply in my build, but with the exception of battening every joint (and I do mean all 8 of them!) for safety and ease of build I haven't added any further bracing. I don't think the cabs have ever been driven particularly hard, but I've never heard any issues that I'd put down to panel vibration/resonance. To be clear, I haven't noticed any issues at all, and they do sound excellent to me! Out of interest regarding commercial cabs, I did have a quick poke around with a camera in the Purple Chilli 1x12 I had for a while and couldn't see any bracing in that, although it's possible there was some hidden by the wadding. The Ampeg PF 115 HE I have has only a single batten bracing the rear panel...

Given the lack of bracing in my existing cabs (which might be a problem if driven harder as a single), and the appeal of having something you can perhaps hear a bit better when you're standing almost on top of it, I'm quite interested in building this new version for rehearsals and perhaps those gigs where, for ease of access, length of set, etc. I don't want to monkey around with two cabs.

Do you have any idea of the target weight for the final design? I think my 1x12's worked out at about 14 and a bit Kg each, which is pretty good. Ideally I'd have something slightly less deep to make carrying even easier, and given the increase in height (or width, depending on how you look at it) for this cab you'll have decreased one or both of the other dimensions - can you confirm which one?

As an alternative to battening every joint, and not owning a set of clamps, I've wondered about using dowels for aligning the panels. I've also thought of cutting the braces from single pieces of plywood - possibly a bit wasteful, but neat, and I wondered if that might increase the strength a little. Any comments on either of those approaches?

Edited by Gottastopbuyinggear

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When I contacted Stevie about this originally I was aiming for a single driver with a weight coming in at about 14KG. However once I heard one of Stevie's cabs I was hooked on a two way.

From my rough, calcs, the tweetered version should come in something under 16Kg and the compression horn version that I am building 17Kg. A good HF unit is weighty and although the P-Audio Tweeter and the Celestiion compression driver are about the same weight, the crossover for the compression horn weighs and costs more as it crosses over much lower than the Tweeter. As I understand it Stevie will publish the twittered design initially. [size=4]Stevie will be able to give you more info though. [/size]

As for the size, the final size is to be confirmed (non) but the prototype was taller than the MK1 (60mm wish )and the other two dimensions were smaller. About 10mm smaller on the width, and 40mm ish on the depth. Sorry to be vague but I built the prototype from 18mm ply and the final design will call for 15mm.

Regarding the build, I will let Phil answer the dowel question but I will be using them.

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