After having built my Parker Audio 98 MkII loudspeakers I decided the next project would be to build a really good subwoofer. The decision was made because I am very interested in making some horn speakers to pair with my little valve amp. I know horn speakers are renowned for lacking in deep bass and that any subwoofer would need to be fast enough to keep up with them.

Being very familiar with Decware, I kept coming back to the site and settled upon the WO32 “Wicked One” subwoofer as a project. It seemed easy enough to build, small enough to fit in the room and also feasible in terms of cost.

To put this in context, I had had a discussion with my ever patient wife about the subwoofer and she offered to buy something for around £450, like a REL subwoofer but instead we both agreed on doing the WO32 Wicked One. What fools!

The cabinet

I purchased the plans from Decware, which are actual full-size printed plans plus a set of notes and a protractor. These arrived in the post and I looked at them longingly, itching to get started. Because my friend John and I had decided both to make a WO32, we set about purchasing enough MDF for both cabinets.

Using the plans we basically went to the shop, picked up enough sheets for the two cabinets, took them to the board cutting guy and asked him to cut them down into the top and bottom pieces and then the remaining MDF in 12″ widths. The subwoofer cabinet is basically two large pieces (top and bottom) and everything between that which forms the front, sides, back and horn labyrinth inside is all 12″ in width so getting the wood cut all to this width makes things quick.

Then we got together with MDF and table saw and basically cut all the 12″ wide pieces according to the plans. This is a fairly simple task of measuring the angles from the plans, setting the saw blade angle, making a test cut on a piece of scrap and then making the proper cuts. Actually we found the table saw was so accurate that we didn’t need to do any test cuts after the first couple. Because the subwoofer has two horn loaded speakers we cut two of every piece (except we cut four of every piece because we were making two cabinets!)

One other important step is to mark the layout of the subwoofer on the base board. Following the Decware plans you make a few markings on the edges at various distances and then join them up producing the amazing X shape of the twin horns.

dry fit

Gluing

Once all the pieces were cut we did a dry fit to check it was all going to work. The pieces went together very neatly indeed so I proceeded with gluing. Over the period of a week I glued together one cabinet, setting up one or two pieces at a time, gluing them and letting them dry overnight. The next evening the cabinet would be ready for the next couple of pieces. When I glued two pieces it was always the corresponding left and right pieces.

I differed with the Decware construction notes when it came to gluing, because the notes said I should stick all the side and labyrinth pieces in at once by applying wood glue to the MDF, then using some hot glue to hold them in place on the base board turn over the cabinet and screw through from the other side. Well “screw that” I thought because all I could see was everything falling off the base board when trying to turn it over. The cabinet is heavy and unwieldy and I do not think it would have worked for one second.

Gluing was very straightforward but one thing I would point out is that, if you are building the same project, you may want to drill holes for the speaker wires before you glue in the relevant pieces. I did this and I think it would have been pretty hard to drill the holes after gluing everything in as you simply can’t get the drill into the space!

I should say that a few things were not glued in at this point; the “velocity discs” and the pieces to which the speaker drivers mount. These boards through which the speaker drivers fire and to which they are mounted need to have a hole cut in them (obviously) so I waited until my Decware DHM-108B speaker drivers arrived, then measured them carefully and cut the holes.

Hole cutting

I didn’t have a fancy jig so I just made one from an 8″ long piece of hardboard. The idea is you just mount a router to a flat plate and make holes at various points along the hardboard around which you will rotate the router. By measuring accurately you can make really good quality holes of the exact dimensions.

hole cut in Piece A (also shows hardwood cleats, see below)

Cutting the holes in the pieces through which the speakers fire (piece A – the start of the horn) is very simple as it’s one thickness of MDF. I cut these holes and glued these pieces into the cabinet.

The speaker drivers are sandwiched between these A pieces and a mounting plate into which you mount the speaker, with some silicon beading around the speaker’s mounting flange to decouple it from the cabinet. My problem was that the DHM-108B drivers were deep enough that there was no way a single thickness of MDF was going to be able to be cut away and hold them with any strength – the speaker mounting flange was practically as deep as one sheet of MDF so I had to glue two sheets together and then rout out the holes into which the speaker would sit. This worked very satisfactorily.

Velocity disc and “wedge” piece

The semi-circular, chamfered, velocity discs and the wedge pieces were, without doubt, the hardest pieces to make but even these were straightforward.

Velocity disc

With the velocity disc, once I had the rear angle cut I just used the router to cut the semi-circular shape. Not having any belt sander, I marked out and then sanded the 45 degree chamfer by hand with a rigid Sandvik sanding block. It was a matter of patience and care to get the final chamfer which was, I think, quite accurate. You must be careful to hold the sanding block flat and not to “round over” the chamfer. Once the velocity discs are glued in you are also supposed to sand over the sharp edge of the piece it butts up against (because the velocity disc has a diameter of 8″ and the internal walls of the subwoofer are 12″ tall you are left with a couple of inches of sharp edge you need to round over).

Velocity disc glued in (same for other side)

The brace (wedge) piece Part F was even easier, just like doing a wood carving – I marked the shape for the top and bottom according to the plans and then sanded by hand until it was just right. (This wedge piece needs to be glued in before piece A).

Final assembly of cabinet

I glued on the velocity discs, glued in the remaining corner strengthening and bracing pieces.

Everything glued and ready for speaker drivers

According to the construction notes, the speaker driver mounting plates are wedged between a hardwood cleat (screwed and glued to the bottom of the cabinet) and piece A. I tried this and it didn’t really work very well because it was very hard to get the cleats tight enough to grip the speaker mounting plate – one of my cleats had a tiny gap which resulted in the subwoofer banging at certain frequencies.

My solution was to instead use eight long screws on each mounting plate which worked brilliantly and when I last checked inside they were still holding very tight. I feel that the instructions from Decware could have been better regarding the chamfer – they probably could have come up with a fail-safe guide to getting it right first time.

To complete the cabinet I screwed the lid on with something like 42 long wood screws. I could have used the plans to mark where to screw but I want to keep the plans intact so I put the lid on and drew around the inside of the horn as far as I could with a pencil, then carefully measured the rest.

Anyway. That’s the cabinet complete! Now you can read about the amp and crossover I built to drive the subwoofer.

Complete cabinet (without lid of course) with speaker drivers fitted

This entry was posted on Wednesday, May 2nd, 2007 at 4:21 am and is filed under Hi-Fi. You can follow any responses to this entry through the RSS 2.0 feed. You can leave a response, or trackback from your own site.