12″ Ported Bass Cabinet. Planning, drawing and adapting a Celestion design.

I wanted to try and work out rough costs for the Celestion bass cabinet design I’m basing my custom build on. Round here – there aren’t too many options where I can buy the 18mm Hardwood plywood I need. I’ve got a small table saw in the workshop – but it’s not really big enough to break down a full 1200mm x 2400mm board. I’m going to have to find a supplier who will cut the board down to size for me.

CabSpec_001

And that’s where a few complications started to present themsleves. The more I read up about cabinet design – the more I realised I needed to build the cabinet to work with the performance characteristics of a particular speaker. That effectively means I have to select the speaker first. It isn’t necessarily going to be as easy as dropping any old speaker into any old box.

Image_00

Although I’m looking at a Celestion designed cabinet – (the design is a ported cabinet, but dimensioned for a standard, 12″ guitar speaker) – I think I’m going to end up with an Eminence, “Basslite” S2012 speaker. Reasons for this are primarily, cost, size and availability. It’s a starting point. Things may well change – but Eminence also provide a whole load of technical specifications, which should help me adapt the basic Celestion cabinet around it. My priority is to build a small, bass enclosure that works well, and provides a good voice for the new 30watt, all valve bass amp.

To begin with – the Eminence bass speaker has a deeper frequency response than the Celestion guitar speaker. It’s what you’d expect, and want, from a bass speaker. Lower frequencies require the movement of a lot more air, and a sturdy, air-tight construction is essential to get the best mechanical performance from any speaker driver – but especially so, if it’s for bass. I’m already considering building the cabinet box with a bit more reinforcement around the frame. To allow for a fully air-tight structure, and to strengthen the carcass, I’ll be using rebated, glued and dowelled joints throughout. Looking at the Eminence specifications, I’ll also require a second port to allow for the movement of the correct volume of air, in and out of the cabinet. Bass speakers move a lot of air.

The Basslite S2012 can work in either a fully sealed, or twin ported enclosure – but I’m hoping the use of a ported enclosure might help maximise the bass response. This is, after all, only going to be a 12″ speaker cabinet – so I need to do all I can to help bring out the lower frequencies. Eminence provide an ideal scenario specification for a generic, “small, ported enclosure”, and recommend a total enclosure volume of 42.5 – 85 litres (1.5 – 3.0 cubic feet). I’m going to have to make sure I can adapt the Celestion design to suit the Eminence speaker, and then make sure I can get the wood supplied and cut to the correct sizes.

Image_01

The simplest thing to do, is to go back to design principles, and draw it all out to scale. Luckiliy – I trained as an Architectural Draughtsman. I still have a whole set of graphic design and CAD tools at my disposal, (and I even remember how to operate some of them).

The Celestion cabinet design is a simple box enclosure made out of 18mm Hardwood ply. I’m going to improve on their basic joinery guidelines, and rout a 5mm rebate onto the ends of both the top and bottom sides of the carcass. The two side pieces will sit in these rebates, and the frame will then be glued together – with glued, dowelled reinforcements at regular spacings. All the edge joints will be reinforced with 18mm battens – glued, screwed and dowelled. The front baffle will be screwed, glued and, (perhaps), dowelled into place. The back panel will be removeable, but held securely in place with coach screws – securely sunk into the 18mm reinforcement battens.

Cutting the side joint rebates will lose me approximately 10mm in terms of height, and will therefore slightly reduce the available, internal volume. From a scale drawing of the carcass, viewed from the front – I can extrapolate and draw the required panel sizes to scale. Drawing them aligned to the front view, but “exploded” out to the sides, and above – all helps to visualise the project, and will help when it comes to explaining the cuts I require to the timber supplier. Now I can work out the internal volume of the cabinet. It works out at 52,200 cubic centimetres. I remember from Physics lessons, that one cubic centimetre equals one millilitre of volume. There are 1000 millilitres in a litre – so our volume is 52.2 litres, (1.84 cubic feet). That puts the enclosure easily within the recommended tolerances specified for the Basslite speaker. Even with some additional “egg-box” foam baffling taking up some of the internal volume – there should still be more than the minimum, 42.5 litres of moveable air within.

Image_02

I can also use the drawing package, (Adobe Illustrator, in this case), to look at the layout of the front baffle board, into which circles will be cut – for the speaker and port tubes. As I’ve read up a little about speaker design – there seems to be quite a bit of thought, and technical theory, put into all of the various dimensional relationships. It really isn’t as simple as sticking any old speaker in any old box. Mathematical formulae will help to determine the exact, required lengths of the two port tubes, for example, – since the enclosure will, eventually, require proper “tuning”. More of that, later on in the project.

For now – the proportions of the cabinet design provide three main x,y and z, internal  dimensions – 500 x 360 x 290. None of the dimensions are the same. They’re different – but they look related, and almost proportional. Time for a bit of mathematical gymnastics. The internal width – 50cm is 1cm more than 49cm, (which itself can be expressed as 7 x 7). The next dimension, 36cm, is 1cm more than 35cm, (which breaks down as 5 x 7). The final dimension, 29cm, is 1cm more than 28cm – (4 x 7). So, effectively, the proportions of the box have an approximate relationship: 7 x 5 x 4.

These proportions are important. In vibrating and moving air to provide sound waves, the speaker works, pushing air both out into the room as the speaker cone moves forward, but also back into the speaker cabinet, as the speaker cone rebounds. In effect – the speaker cabinet is “tuned” to try and ensure that rebounding waves, and the sound frequencies produced within the cabinet can’t interfere with each other. This can provide unwanted phenomena such as phase cancellation, or resonant feedback. The proportions of the cabinet appear designed to specifically reduce these unwanted effects. If I want to change anything about the design of the cabinet – I probably need to make adaptaions which fit around this basic, dimensional relationship.

The placement of the port and speaker openings also seems to follow a similar, mathematical logic. In the Celestion design, the speaker isn’t just mounted centrally within the cabinet – it’s actually placed just slightly higher than centre. As it turns out – once I had drawn the dimension of the Eminence speaker frame onto the drawing, I noticed that the securing bolts would probably foul the extra carcass battens I need for the front baffle.  I therefore needed to move the speaker down slightly. The original design required the centre of the speaker opening to be located 192 mm above the internal base of the cabinet, and 176mm in from the left. I realised I needed to lower the speaker by 5mm, so that the dimension above the base would now be 187mm. In order to keep a bit of difference between the two mounting dimensions, and in keeping with the overall, apparently designed, eccentricity – I moved the opening to the right a little bit more – to 196mm. This so I could locate the centre of the speaker opening 196mm in, 187mm up (and therefore 173mm down). That’s roughly 17.5 – 18.5 – 19.5. It almost looks like I’ve planned it all along.

Since I moved the speaker opening, I also needed to move the port over a bit. In fact – I needed to move the port, and also add a second port. (The Eminence speaker specification for the generic, “small, ported enclosure”, requires two, 3 inch ports – one mounted flush, the second tuned to length).

The original, single port was relocated so that the placement of the centre of the opening now fell at 98mm above the base, and 108mm in from the right. The openings themselves need to accept 3″ tubes (77mm). When I looked at the problem – I just thought that the sequence 77 – 98 – 108 was clearly missing 88 and 118. When I placed the second port 118mm down from the top, and 88mm in from the right – it just looked right. Although the placement of each of the openings is eccentric – the three together just seem to balance visually. Of course – this is no guarantee as to what it’ll sound like – but it goes some way to satisfy the graphic designer in me, and it’s interesting to discover some visual and numeric “balance”, in there.Image_03

So, now I have to work out how much wood I’ll need. I also need to keep the number of cuts required to break the sheet down, to less than 17. For some reason or other, that’s a limit my local B&Q warehouse insists on. The cutter at the warehouse is laser guided, straight and accurate. I’m sure they will do a decent job, and will cut the board to millimeter tolerance – but I have to assume the board will need squaring first – so there’s the first cut of the 17. Laying out the other panels required, based from this first, squaring, cut – I figure it’ll take 10 cuts to produce the six pieces of ply required for the cabinet. At £38.00 for a single sheet – that’s not so bad cost wise. But there’s a whole, half sheet going begging. What a waste. Checking with the supplier – I can’t find an alternative sheet size which will work as well. For the money, I might as well get enough panels cut for two cabinets, and maybe look at building a second as an alternative project – maybe with a different driver in place.

Image_04

Satisfyingly – when I lay out the panels for a second cabinet – there’s still plenty of spare wood. Annoyingly, however – it’ll take 19 cuts to reduce the sheet down. 2 more cuts than my allowance, (only one if you don’t count the first, “squaring” cut). When I spoke to the guy at the warehouse – he was adamant that they wouldn’t do more than 17 cuts. “We have all sorts of tradesmen in here asking us to cut their entire kitchens for free!” I’m just going to have to appeal to their better natures. If they won’t play ball, I’ll just have to get the best I can with 17 cuts, and then make the two extra cuts myself – the best I can do – with a handsaw,  back in the workshop.

Oh – and then one more problem. The special laser guided saw? – Broken. Out of comission for another “week or so”, until they can get a repair man in. With my Dragoncaster body currently drying – in between coats of nitro clearcoat, and with parts for my Black Strat stuck in customs, awaiting clearance – for once, it looks like I’m scratching around for projects to work on.

Leave a Reply

Fill in your details below or click an icon to log in:

WordPress.com Logo

You are commenting using your WordPress.com account. Log Out /  Change )

Google photo

You are commenting using your Google account. Log Out /  Change )

Twitter picture

You are commenting using your Twitter account. Log Out /  Change )

Facebook photo

You are commenting using your Facebook account. Log Out /  Change )

Connecting to %s