I wanted a cheap and powerful subwoofer for my speakers and instead of shelling out £100s I went for the DIY route after finding a 10" Pioneer TS-W255C for just £10 used. I'll get onto how its powered later on

To make this you need:

Old Computer Power Supply (Power depends on how big your sub/amp is)
Car Amplifier (Something which is bridgeable and has a built in crossover is best for this)
Subwoofer driver, there's so much choice here but a lot of the car subs on the market are very cheaply made, look for a heavy magnet over fancy casings
Speaker terminal, very cheap from sites like ebay
18mm MDF, also depends on the size of your sub but this is the material of choice for the vast majority of speakers
Lots of sandpaper, primer and spray paint
Some nice thick speaker wires (Don't have to be expensive at all)

Ask if you want more detail on any part of the build

Step 1: Design

To design and plan what kind of box I needed I used Google Sketchup to design it after working out what volume and port size/length I needed in WinISD. This gives the woofer a usable extension down to around 32Hz while keeping the box a nice 35cm/13.7" cube enclosure.

As you can see the port has a corner in it, this isnt idea but well worth it to get the lowest possible response

What your looking for in WinISD is a flat response with the lowest response you can get. Its a pretty advanced program so you are best off watching a few videos online.

Remember the volume of the port is excluded from the volume of the box (Add port volume and box volume together then calculate dimensions)

Resources

WinISD - To use this you have to know all of the thiele small parameters (T/S) of your speaker

For some reading on speaker boxes http://www.bcae1.com/spboxnew2.htm http://www.the12volt.com/caraudio/boxcalcs.asp#por...

As pointed out in the comments wall thickness of the port as well as a rough estimation of the volume the driver will take up need to be taken into account and a not part of the "Box Volume" WinISD will give you

Step 2: Cutting the Wood

Pretty simple step, cut down the 18mm MDF and do a dry fit to check it.

Next the cutout for the speaker needed to be made along with the terminal for the back of the speaker. I used neoprene as its super cheap as a gasket for the speaker which I just cut out with a stanley knife, thsi gives a air tight seal around the speaker and the box

Step 3: Next Came the Port

This is where it gets a bit interesting as using 110mm guttering is the best I could do for my port to get it big enough and long enough. If I had more materials in a perfect world I could have designed the box to hit a bit lower than 32Hz but hey.

Other options are slot ports and round ports, this is just a case of what fits best

Step 4: Then Came the Glue

Lots and lots of wood glue then left in clamps overnight. I opted for no nails or screws just to make my life easier when i came to painting and it seems to have done the job perfectly

Step 5: Port Fitting

Cutout the port before gluing the used sealed it in with adhesive and silicone.

At this point I probably should have reinforced the loose end with some kind of cross section to prevent vibrations but it seems pretty sturdy. To give a sense of scale you can easily fit you hand in the port

Step 6: Router

Ran a router and sanded all the box to give it a nice round edge. That picture shows the size of the woofer pretty well and its 32oz magnet

After it had been routered it needed to be sanded, I used a palm sander with increasing grain sandpaper until it was as smooth as I could get

Step 7: Painting

Primer (First of very many coats) then after so much hassle and a fair bit of shouting the paint was done to an acceptable level (Used black automotive spray paint). It was a real bugger to get a nice smooth finish on this but I'm pretty happy with it, the finish really relies on the primer

You're best off doing a coat of primer, leaving it overnight sanding then repeat this with finer sandpaper until the finish is smooth to the touch. This (should) give a nice gloss finish after a few coats of spray paint

Step 8: Dampening

Don't actually know what this stuff is meant for but I guess insulation, perfect for this as well though. Used a staple gun to secure it all down on every side of the box and made it as light as possible by pulling it apart.

This should make the box less boomy as standing waves and resonance is limited resulting in more punchy bass but too much can take away from the volume of the box and mess up your calculations

Step 9: Powering It

All the electronics.

A 500W PSU from a old PC with all the 12v and ground wires bundled together, then using a 2000W bridgeable Lanzar Heritage car amp which powers the sub very nicely

Simply bundle the green wire on the PSU in with the ground wires on the amp and if your amplifier has a REM attach it to the 12V so the amp will turn its self on with the power supply

EDIT: I have gone into this in more detail in the comments

Step 10: All Done!

This thing really kicks out some bass and the 11mm xmax is pretty goddamn impressive to watch

Step 11: One of the Many Places It Lives

Overall this was a very cheap built with the driver only costing £10 and a used £30 amp.

Step 12: Very Happy Overall Even Though the Gloss Finish Drove Me Mad. Feel Free to Ask Questions Below, Im Sure I Missed Plenty Out

A bit of flex, it can do a fair bit more than that but stuff in the room starts shaking far too much. Really gives that thump in the chest for movies

Possible improvements:

Slight port adjustments, at full volume the low notes shift so much air there is some chuffing but isn't a big problem
I have feet to add to it but keep changing its orientation but this will help with the vibrations
Recess the driver, If I doubled up the MDF on the front I could have the driver nearly flush instead of sticking out and it would improve strength
Include the amp and PSU inside the enclosure, for me this would be more effort than worth because I like to be able to tinker with crossovers and check temperatures ect but it would look cleaner

Hope you enjoyed my first ever instructables!! Please consider me for the contest at the top, Thanks