Tannoy Monitor Gold 12" build.

[awkwardbydesign]

I tested them again with my big old valve sig gen (enough power to drive the speakers direct)  and the trusty old Tandy SPL meter.  The wrong 'un WAS wrong!  Rewired the right way round and the dip has gone.  So I can just play them and run-in the crossover caps while I work out the cabinets.

[awkwardbydesign]

Just had a draw. 1200mm height, 500 front, 320 back, curved sides gives me 160 litres.  Add in braces, back of speaker, crossover, felt pads, ports, generous round-overs for the baffle edges and I will still be above the 135 litre target.  So I can price up the materials.  Unfortunately not Baltic birch, as my local timber yard doesn't sell it, and I get a good trade discount there.  It'll be fine! 

[awkwardbydesign]

I'm actually considering kerf cutting 18mm ply, filling the cuts with polyurethane glue, and then skinning it with 4mm hardboard plus 4mm birch, and gluing those with non-setting acoustic adhesive, to get the smooth outer curve.  Mainly for cost reasons (6 layers of 4mm birch is a very expensive way to go), but also it would be slightly easier.  I've used a mix of birch ply and hardboard before in curves, and it worked well, IMO.  Hardboard seems to be a forgotten material, but I rather like it.  Cheap too!

[Reffc]

It seems a lot of work Richard when the same effect can be had using MDF, cutting slots into it to effect a radius and then skimming the outside of that with 4mm birch or even easier, a structural Maple veneer or similar (veneer of more than 1.2mm thick). 

[awkwardbydesign]

I'm not a fan of MDF, having compared the sound of identical cabinets in MDF and 700kg/m3 flooring chip in the '70s.  I will use it when I must, or mix it with birch ply, but not alone, at least not for serious speakers.
I will do it with the birch (kerf cutting) for peace of mind. 

[Reffc]

It's actually not bad at all as long as internally panel damped using iso-elastomer sheeting and laminated with a thinner ply outer.  On its own it's not great for large speakers as structurally it is not sound....easily damaged.  Mind you, most folk are unaware that there are many grades of MDF, some coming in close to 800Kg/m3 and much denser than run of the mill building fit-out stuff which is not so good for cabinets.  It's hard on tools though...

[awkwardbydesign]

And lungs.

[Reffc]

Yes, that is true.  I always wear a mask and use dust extraction when cutting or routing it though.  Personally, I use Latvian Birch ply with a design density (due to the ply layer  thickness and resin type and amount used) of 18Kg/m2/18mm sheet thickness which here equates to furniture grade AB ply.  Not all plys are the same and it's worth searching out the right spec boards.

[awkwardbydesign]

I think I have finally figured out the dimensions to suit the required volume, domestic acceptance, and cutting economy.  But it could all change!  Externally, 1100mm H x 500mm W (front panel) x 420mm deep.  The height suits the 1000 x 500 x 10mm grille foam I have.  Rounded side edges (50mm radius), probably using PVC pipe, rather than planing the cabinets.  I get around 140 litres after deducting speaker and brace volumes, so a little to play with above the 135 litre target.
And about £140 in ply and MDF costs so far.  



But where do I get the T-hook Velcro for foam grilles?  I have a few left over lengths, but not enough.

[awkwardbydesign]

I've been busy with other things, but now my local timber yard hasn't got any 4mm birch ply.  And they don't know when they can get any more!  Apparently there is a shortage or something.  The nearest suppliers seem to be Falmouth or Bristol, and both of those would make it quite a bit more expensive.  So I had a practice (with a nice new saw blade) cutting separate staves, like a barrel, and gluing them together.  it seems to work quite well, and then I will skin them with, probably, 3mm MDF inside and out.  I think I will buy the 18mm birch tomorrow, just in case that sells out!  Plymouth is a desert for material supplies, unless you want whatever Wickes, etc, happen to sell.

[awkwardbydesign]

Final height - 1120mm, to allow for a 20mm top lip, leaving 1100mm for the foam grilles.
I have cut all the staves, with 3.75 degree edges, like a barrel.  First one stuck and dry, second one drying.  3mm hardboard inner skin, 3mm outer will be stuck on after construction, to get it all the way to the front and rear edges.  I used a basic PVA, as there is such a large area being glued that I don't think I need a more expensive one.  They are a bit taller than the QUAD 2805s, and will be taller still with their bases.  I haven't decided where exactly to position the drivers on the baffle, as I would like to get the acoustic centres as high as I can, but there are other considerations.

[ian]

Great skill!

[awkwardbydesign]

A little more progress.  All four sides made, and one pair screwed to the top and bottom plates.  The curves want to "relax" slightly, so I am concentrating on getting them held in place before moving on.  The other one will be screwed together tomorrow, and then the braces will be fabricated and screwed in.  No gluing until all the flat panels are laminated up to their final thicknesses.  But it's heavy already!

[lapierrerider]

Are you sanding the flats into a smooth curve. I have curved MDF by cutting slots from the inside then glue in the slots and pull the curve round

[awkwardbydesign]

They will be covered with 3mm hardboard, which will form a smooth curve, the same as the insides are.  I considered kerf cutting, as you did, but the force necessary to bend that size panel was excessive.  And there would have been voids in the cuts, so cutting and gluing staves was easier, for the same effect.