12″ Ported Bass Cabinet. Rear panel and sound deadening foam.

With the contact adhesive from the previous day’s covering, now dry – I clean off the odd bit of residue, with a quick wipe of cellulose thinners. This definitely softens the vinyl. I need to find a better alternative. (Note: Nail varnish remover – acetone – seems to be the answer). I remove the handles and jack plug socket again – noting that they’ve succesfully clamped the vinyl down around the openings, while the adhesive has dried.

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The panels for the baffle and the rear panel were cut to fit the bare wood cabinet exactly, and they’re extremely snug. So far, this has helped when it comes to putting the box together – but now both panels need to be trimmed a little, to allow for the various thicknesses of Tolex, which now wrap the cabinet. The back panel, itself, will soon have an extra couple of thicknesses of vinyl to deal with – so I estimate the panel needs to be trimmed down by approximately 5mm in both dimensions.

To keep things nice and straight, and the cuts perpendicular – I’ll be cutting the panels down using a router. I set up the router with a straight cutting bit, and clamp a straight edge down, so that the router just follows one edge. I can then reposition the straight edge 5mm further in, and then use the router to follow the straight edge, and trim the edges accurately – keeping everything square. For now, I’m only dealing with the back panel – the baffle may, eventually, require a similar approach, with a slightly different fitting – so I’ll eventually trim that in the same way.

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Because the Tolex will be wrapped around the edges of the rear panel – glueing it, is going to be a messy business – so it helps to plan ahead and draw out where the screw holes, and fabric edges will land. I don’t want glue simply everywhere, if I can help it. Because I don’t want the securing screws to push through the Tolex on the inside of the panel, (they’ll probably pull the Tolex away from the wood, with repeated use), I need to trim the Tolex accordingly. That means locating the screw positions in advance, and trimming the outside edge of the Tolex piece to match. The corners are also cut – so that they mitre nicely, when folded around the panel.

Once the Tolex is trimmed to the required size and shape, I drill the screw holes with the correct size bit,. I’ll be using 40mm long, 4mm diameter, stainless steel screws – so I use a sharp, 4mm wood drill, and then give the panel a light rub over with a coarse, Scotchbrite pad – to make sure there are no loose splinters or other bits of wood to telegraph through the Tolex.

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I also add small strips of black Gaffa tape, to the edges of the panel. The edges will be just visible in the reveal on the back of the cabinet – and so if the Tolex covering at the corners doesn’t exactly fit – then the wood will show through. (I should have probably done this on the internal corners of the main cabinet – but it didn’t occur to me then. I covered up the odd bit of wood that showed through there, with a bit of permanent felt-tip. Using a black, fabric tape beforehand seems a bit more professional).

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Once I’m sure everythying has been measured and marked properly – both the reverse of the Tolex, and the panel, are sprayed with contact adhesive. Both are then set aside for a few minutes, until the adhesive becomes tacky. The wooden panel is then placed dead centre, (follow your guidemarks), the panel is flipped over, and the fabric rolled from the centre, out to the edges. Then – with each edge positioned, in turn, over the edge of the work bench – the edges are stretched over and rolled. The panel can then be flipped over onto its’ back, and glue can be applied for the edges. Once the glue has “tacked-up again – the edges can be stuck down. With care, you can do all this without getting spray glue everywhere – although it helps to take your time, and plan ahead. With the panel forming a kind of tray – it’s easy to place a little bit of extra protective covering down onto the workbench – to prevent any subsequent overspray.

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Adhesive can now be sprayed around the edge of the panel, as required, so that the remaining, triangular “tabs” can be stretched, pressed and, finally, rolled into position. Gently heating the edges with a heat gun can help to encourage the vinyl to adopt its’ new shape – but you only really need a gentle warm. Don’t over-heat the fabric, or over-soften the glue. And make sure every surface is rolled and secured.

While the glue now dries, I mark through each screw hole with a bradawl, from the back of the cabinet. The holes in the Tolex can then be located on the covered face of the panel and widenened, before the Tolex is re-rolled, and pressed back into position. I find it helps to press into each screw hole with the tip of a pencil, while the glue is still drying. It helps to locate the holes again later, and seems to help secure the Tolex around the edge of each drill hole.

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The areas on the inside of the panel, where the contact adhesive is left exposed, soon dry – and to keep things tidy, I run a line of Gaffa tape along each inside edge – mitreing each corner – so that it neatens up the overall look, and secures the glued edges. After a final check over – cleaning up any overspray as necessary – the rear panel is put aside, until the glue is totally dry.

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I have sourced some 30mm “egg box” acoustic foam – to line the inside of the cabinet. This should deaden any internal sound reflections, and ensure that I’m hearing the right sounds from the speaker, once it’s fitted. I measure each internal face of the cabinet – between the reinforcing battens – and cut suitable panels of foam from the roll it arrived in. It’s easily marked with a pen – and can be accurately and neatly cut with a straight edge and a sharp, new, razor blade. Hold the blade straight and perpendicular, and cut in one, smooth stroke – without compressing the foam. You may need a fresh blade every now and again. The foam quickly dulls even the sharpest edge.

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I fit the side panels first – laying each in place, and then securing them in position around the edges with 8mm staples from a staple gun. The cut outs for each handle, and the jack socket, are traced and cut through from the outside, with a long-bladed knife. I then staple around the edge of each opening – to securely fasten each piece of foam in place.

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Similar pieces are then placed inside the top and bottom panels of the cabinet, and are stapled into position. It helps to cut them to the exact length of cabinet interior – rather than to the visible length left after fitting the side foam pieces. This ensures a good meeting at the edges – a kind of compressed overlap. You can push each panel into position with your fingers, and let the foam compress naturally, at the corners.

Once the foam is in position – the handles and jack plug socket are screwed into position again, and the screws torqued down. It’s coming together.

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A suitably sized panel of foam is then cut to fit onto the rear panel. I stick it into place using heavy-duty, double-sided tape – but still add a few securing staples – as before. I cut the piece short of each edge – so that the panel will fit snugly, between the securing battens. With the panel now ready to offer up to the back of the cabinet, I can test the fit.

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The rear panel fits nicely within the opening and, without the screws, sits slightly proud of the rear of the cabinet sides. However, since it’s sitting on the rubber gasket I previously installed, it should bed down into position once I torque down the securing screws. This promises a good, air-tight seal, and the fit looks consistent, all the way around.

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The 40mm securing screws are then located in position, and tightened – through the rear panel, and into the softwood battens beyond. Each stainless steel screw has a cup washer. I located all of the holes earlier – they’re easy to find. I work evenly across the panel – from side to side, and diagonally across – to opposite fixtures. Working out from the centres of each edge – towards the corners. I want the gasket to compress evenly.

It’s probably overkill – but I looked for a regular screw spacing which would work with both dimensions of the back panel. I also needed to take into account possible clashes with the screws securing the battens, inside the cabinet. In the end, I came up with a spacing at, approximately 55mm centres – so that each screw sits between each of the internal batten screws. But that’s 30 screws all together. Seems like a lot for such a small cabinet. Once the screws are in – you can certainly feel the weight of the additional metal. With luck it’ll help balance out the speaker! It might well be overkill – but that back panel certainly isn’t going anywhere.

And that just about completes the cabinet carcass. All I need to do now, is source the speaker, port tubes and a suitable grille cloth – and then I can look to fit out, and install the front baffle.

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