I decided recently that my home cinema system could do with a nice subwoofer, and rather than buy something, I thought I'd build myself one...

NOTE: This is only a short explanation, there is far more info here - http://www.yourmissus.com/subwoofer/

Step 1: Cutting

Construction starts with measuring and cutting the MDF.
My design with its double skinned walls does require quite a few panels - 18 in fact, and to ensure we end up with a rigid and airtight box the cuts must be very accurate.
Ideally for this task you would use a tablesaw - we are looking to achieve accuracy of panels to better than 1mm.
This would take some doing (or at least a lot of plane work and sanding) with a handsaw or a circular saw.
This first stage of marking out and cutting is very important (and a little boring) but it cannot be rushed.
The end result really does depend on how accurate these panels are cut, so measure twice, cut once, take your time etc...
It will pay dividends later.

Step 2: Glue and Screw

Using plenty of glue is key to a good strong assembly, ideally the woodglue will ooze from the joints when the screws are tightened and should form a continuous bead - which is a good indication that enough glue was used. When gluing I tend to use small paintbrush and coat both mating surfaces with glue.
Once assembled though I don't think it hurts to brush just a little more glue into any remaining gaps, I try to get the glue as deep into the gaps as possible using a wallpaper scraper to push it in under pressure

Step 3: Ports

The inner port calculations reveal that the port must be 271mm, this is fairly tight considering that each chamber is just 149.5mm wide plus 36mm for the centre baffle giving a total of 335mm
Assuming we arranged the port so that it was centred on the baffle that would only leave 32mm at each end (less once we fix some wadding/sound-deadening at each end)
It is generally accepted that the ideal is to leave at least one port radius between the opening of the port and any surface. Unfortunately with a straight port located tight in the corner we can't achieve this, so the plan is to bend the port a little.
I suppose it doesn't really matter how you do this as long as some attempt is made to maintain a fairly uniform internal cross section.

Step 4: Gasket

I want this lid to be removable for the purposes of tuning the enclosure, so there needs to be some kind of airtight seal where the lid meets the box, my plan was to route out a semicircular groove using the router and then apply bathroom silicone to the groove to form a gasket - I had no idea how well this was going to work out!!

Step 5: Drivers

The speaker wires come through the baffle and then pop out near to the driver connection tags, the picture below shows the wires connected up. I soldered them to the speaker tags as this is more reliable than crimping connectors on, as always a couple of cable ties and P clips help hold the wire in place, I've also used cable ties to show which wires are +ve and which are -ve.

Step 6: Electrics

There isn't really too much to do electrically, just link up the speakers to some kind of external connector.
I favour the neutrik speakon connectors, they are capable of carrying huge amounts of current, they lock into place very positively and have the added bonus of being 4 pole:
I've used a right angled 4 pole neutrik plug, as the look quite neat, and some 4 core 4mm sq layflat oxygen free copper speaker cable, it is probably the largest cable you could get away with using with this plug, so you do have to be careful with the connections or else you'll run out of space

Step 7: Final Assembly

Then applied plenty of glue to the top of the rectangles. I then placed the base on top and once I was happy it was in the right place I put a few screws in to hold in and then checked that all was well before finally screwing it into place.

Step 8: In Use

Of course I'm pleased with my efforts, I really did put a lot of effort into this, so failure really wasn't an option!
It's been in the lounge for a few months now and is fairly well worn in, and certainly it makes bangs and bumps at all the right times!
A bit of initial messing around with the levels to get it balanced in the mix was required (I use a separate power amp to drive this one) but once that was out of the way it's been plain sailing really.

NOTE: This is only a short explanation, there is far more info here - http://www.yourmissus.com/subwoofer/

Thanks for your interest
-Dan