One of the best ways to save some money on your stereo system and still get the best sound is to make your own speakers. There are many directions to go; multiway (using more than one driver per speaker) or single driver, and then there are the cabinet designs: open baffle, ported, horn loaded, sealed, and transmission line are the most common types. Because of the overwhelming number of directions a speaker build can go, it's important to start with a budget, and then take into consideration your listening requirements. Listen to hip-hop and host parties? Ported with some big woofers (low frequency driver) is the way to go. Like classical at softer listening levels? Probably open baffle with a single full range driver. Small listening space? Keep the drivers and cabinet on the smaller size. No 18 inch woofers for you. Since the design of the speaker is entirely in your hands, it's possible to tailor it to your needs.

For my build, I gave myself a budget of 250 dollars for the pair. This seems extremely low compared to the price of store bought speakers, but it really isn't too limiting because of the extreme markups on commercial speakers. For my design, I chose to go a single full range driver in a MLTL, which stands for mass loaded transmission line. The basis of transmission line theory will be covered later. This design was chosen because of the simplicity of the build, lower cost, and the better soundstage, low distortion, and fidelity that a full range design offers. With this design goal in mind, the WiBAQ was born. It stands for Wild Burro Audio Quarter wave. Wild Burro Audio is the manufacturer of the driver I used, and it's a quarter wave transmission line speaker. The name is a play on the TaBaQ design by Bjohannesen on the diyaudio.com website, who helped with the modeling of the speaker and who's TaBaQ the WiBAQ is base on.

Step 1: The Design

So, the WiBAQ. The official WiBAQ thread is here. But the original inspiration is in a different thread, linked to on the WiBAQ thread. The speaker utilizes a Wild Burro Audio Labs Betsy K full range driver, which is available through Wild Burro Audio for 110 dollars a pair. For their performance, these drivers are some of the best value on the market. The driver is mounted in a mass loaded transmission line enclosure. This means that the port at the bottom adds resistance to the air moving in and out of the cabinet, which is being pushed/pulled by the driver. This effectively increases the size of the cabinet. This is similar to how a standard ported speaker works. But in a transmission line, the length of the "line" or "pipe" behind the speaker is one quarter of the tuning frequency. Here, the mass loading of the port and height of the speaker together tune the enclosure to 35 Hz. This gives the enclosure an f3, (or a 3db cut off, or the point where the volume at that frequency has decreased just enough to be noticable) of 35 Hz, which is actually pretty low, especially with a speaker of this design.
Pictured is a Google Sketchup model of the speaker. The dimensions given are the inside dimensions. Its the insides that are the important ones, since its the inside ones that dictate the tuning of the enclosure. The external cutting dimensions of the enclosure will be given later in the Instructable.

The first PDF is the original design with the driver mounted 15" from the closed end. This was later changed to 9" from the closed end and the new frequency response chart included in the second PDF. All the information in the first PDF still applies when the driver is 9" from the top except for the frequency response. In these PDFs you can see the ruler flat bass response, with no peaks or humps, as well as the high 95db sensitivity

Step 2: Materials and Parts

So lets start buyin stuff! The material that should be used for the enclosure is 3/4 inch MDF or plywood. Plywood, even though it is slightly more expensive, can be finished if cabinet grade plywood is purchased. MDF, on the other hand, has to be veneered or laminated for the best finish. For this reason plywood was chosen for my WiBAQ.
If you go with plywood, make sure you purchase cabinet grade plywood. This will have a nicer veneered surface. Also, check the edges of the wood. If there are many large gaps between the separate pieces that makes a ply of the wood, don't purchase that piece. You want as few voids and gaps as possible.
The next obvious component is the driver. The Wild Burro Audio Betsy K can be purchased through Wild Burro Audio Labs here. You have to contact the owner and he will take care of you.
Along with that, you will also need to purchase binding posts, wire, and cabinet feet. All of this can be purchased through Parts Express.

A more specific parts break down:

2 sheets of 3/4" x 4' x 8' plywood or MDF
2 pairs of binding posts and binding post plates (PE)
2 sets of cabinet feet (PE)
2 Wild Burro Audio Betsy Ks (WBA)
Speaker stuffing (or poly-fil pillow stuffing) 1.8 pounds per speaker (3.6 total)
1/2" foam weather stripping (Preferably adhesive backed)
Wood glue (construction adhesive that comes in the calk tube is the best)
Finish grade screws (1.5" cabinet screws for the enclosure assembly and #8 x 3/4" wood screws for mounting the driver)

Step 3: Preparation and Cutting

When you go to Home Depot, Lowes, or any other place to purchase the material for your enclosures, you can also get the wood cut. Make sure you go with the cutting plan pictured. Or, cut it yourself with a table saw. The cuts have to be precise to make sure the pieces fit well. When it comes to the hole for the driver, a router with a circle jig is the best bet. I used a jig saw, and the hole was pretty rough. Thankfully, the driver mounts from the front, so the rough edges and uneven hole shape is hidden.
The hole should be 7 3/8" wide.

If you choose to cut the wood yourself, a table saw should be used with guides for the plywood. If you don't have guides, like me, then your best bet is to use a circular saw and a straight edge to make the longer cuts and then use the table saw to make shorter cuts.

The cutting plan is attached. When cutting, make the long cuts first, so you have 8' strips, and then cut these to the needed dimensions of the pieces. I found this was the easiest way to cut the large sheet of plywood, and would lead to the pieces being most uniform in size.

Step 4: Assembly

Assembly is straight forward. It is recommended that cabinet screws or other finish-grade screws are used along with glue to assemble the cabinet. This means that work can continue, even if the glue has not dried fully.

Start by attaching one side panel to the back, and then the bottom panel. Next, attach the top panel and the front, and then the port piece. Then, add the bracing, wiring, and stuffing. Attach the other side panel last. It is important this order is followed, since proper positioning of the panels can only be done in this order. The general idea is to build the entire speaker, leaving one side off, and attach that side last. This makes access to the inside of the speaker easier during assembly.
Also, make sure edges are aligned properly, and corners square. If the cuts were clean and careful at the beginning, it makes this part easier. When assembling, put a bead of glue the entire length of the joint (you want it to be air tight) and then drive a screw through to clamp the joint together.

Some other notes for assembly:

The bracing pattern is not particularly important, as long as the bracing does its job of stiffening the speaker cabinet, and does not obstruct the sound waves from propagating up and down the enclosure. For my speaker, one 47" by 3/4" by 1.5" strip of plywood is glues corner to corner across each of the side panels, so it goes from the back bottom corner to the front top. Also, placed 17" from the top going from the front panel to the back is a 16.25" by 3.5" piece of ply, and a second 11" by 3.5" piece of ply going from side to side, positioned 17" from the bottom of the speaker.

Before stuffing is added, drill the holes for the binding posts on the back of the speaker near the bottom. If the ones listed are used, two 7/16" holes can be drilled for the solder pads and biding posts to be inserted into.
Also drill the holes for the driver. The best way to make sure the holes are lined up is to place the driver on the front and center it in the hole, and mark the holes for the screws around the driver. Make sure the holes are properly centered, so the screws can be inserted and not contact the rim of the basket or the gasket round the speaker.

String should be added where the stuffing is going to rest. The stuffing is placed in the top 2/3 (30 inches) of the cabinet. 30 inches from the top, a net of sorts made of string stapled to the sides of the enclosure should be placed to prevent the stuffing from falling to the bottom of the enclosure. Strings should be strung horizontally in the upper 2/3 of the cabinet to properly support the stuffing to prevent the stuffing form compacting near the bottom net. See pictures for more details.

While the cabinet is on its side, also drill the holes for the cabinet feet following the instructions on the package. Thread in the cabinet feet holder thing, but do not screw in the spikes themselves. Keep these off until the speaker is ready to be placed in its final position, because moving the speaker with the sharp spikes attached is asking for trouble.

When wiring the driver to the binding posts, use solid core wire. Two or three strands twisted together for each wire is preferable over just one strand. A good place to look for some good wire is CAT-5 cable, which can be purchased in bulk for cheap from places like Home Depot or Lowes.

The mounting of the driver can be done when the last side is not attached, or saved until it is. Before mounting the driver, wrap a piece of 1/2" weather stripping around the flange, so that when the driver is actually inserted into the whole the metal basket won't actually touch the wood. Keep it neat and make sure the ends touch so that there is an airtight seal and the weather stripping does not hang out from underneath the driver in the front when mounted.
When mounting the driver, move in a star pattern to tighten the screws. Start by loosely screwing in the top, then the bottom, then the screw to the right of the top one, and then the screw to the left of the bottom one, then the second screw to the right of the top one, and so on. Keep this pattern going, increasing the tightness on each pass until the screws are snug. They don't have to be massively tight. Just snugged up.

Finishing the wood is up to you. I chose to leave mine bare in the mean time, since it is winter is Massachusetts and I don't want to apply linseed oil in the house. I plan on finishing the wood eventually.

See the pictures for some more detail on the assembly process.

Step 5: Optional: BSC Circuit

Things are about to get technical.

High frequencies due to their shorter wavelength, tend to bounce off the front baffle of the speaker surrounding the driver and radiate forward (called a 2pi radiation pattern). Bass frequencies, on the other hand, radiate around the speaker enclosure because of their longer wavelength (4pi pattern). This means the sound is less focused on the listener, and thus the bass sounds weaker relative to the highs.

To fix this, you attenuate the highs (reduce their volume) to bring it down to the level of the bass. Usually a 6db attenuation is required, but since the WiBAQs have the front port, which reinforces the bass for the listener, only 3db is required.

A simple circuit is needed to accomplish this, called a baffle step correction (BSC) circuit or a baffle diffraction loss correction circuit. It is a resistor and inductor wired in parallel, and then wired in series with the driver. The inductor, for the WiBAQ, is 1 mH, and the resistor 2.5 ohms. The recommended inductor is this. I recommend wiring two resistors in parallel to get the desired value in order to increase the power handling and reduce inductance, so here are the recommended resistors.

Step 6: Use It!!!

The speaker might sound a bit harsh at first, since the drivers need to break in. Given 20 hours of playing time at moderate levels, the speakers will begin to smooth out and sound fuller. If after the drivers are broken in and they still sound a little bright for your taste, and you didn't add the BSC circuit, try adding it.
Also, it takes time for your ears to adjust to the sound of the speakers. Why? That I can't explain. But once you become used to the speakers everything sounds better.

Now, when using the speakers, keep in mind a single driver is trying to reproduce the entire audio spectrum. While these speakers go VERY loud, they are not DJing speakers.

When positioning the speaker, place them so that they point straight out; do not angle them in towards the listening position. With full range drivers, being slightly off axis like this is beneficial. Proper positioning of the speaker can lead to dramatic differences in sound, so feel free to experiment.

Here are my listening impressions before the drivers had a chance to break in and before the BSC circuit was added:
First impressions in three words: Loud, bassy, detailed.
First impressions in more detail: The main selling point of the WiBAQ, going off of the feedback from people here, was its potential for strong bass. It sure delivers. The bass is clear, concise, and boom-free. And deep. Using a test CD I made, it was just possible to feel bass as low as 10 Hz. Around 20 Hz it becomes more noticeable, around 30 Hz its now safe to call 'loud', and by about 40 Hz it reaches just about as loud as it would get for the rest of the frequency sweep. The bass, despite its good extension and it being a mass loaded system, does not have the rumbling and boomy sound associated with most ported systems.
Now for the mids and highs. There is an extraordinary amount of detail, and the highs are well extended, which I was afraid they wouldn't be with an 8" driver. the mids are a bit "shouty" for my taste at high volume, and the highs a bit harsh. I think this is mainly because the drivers are not broken in yet. I bought them used here on the forum, but the seller said he bought these Betsy Ks with some regular Betsys to experiment with. He preferred the Betsys in his Rondo cabinet and only put a couple hours on the Betsy Ks. I will probably put in 25 hours or so of moderate-volume music to break them in before I start doing some more involved listening.
The music I used for this test varied widely. I started with 'Funeral' by Arcade Fire, which is more acoustic and complex in nature. Next I went to the other end of the scale, with 'Of The Blue Color Of The Sky' by Ok Go. This album is extremely bass-heavy, with complex distortions used on the vocals and instrumentals. Songs such as 'White Knuckles' and 'This Too Shall Pass' tend to make speakers fall flat on their face by simply overwhelming them, but these stood up very well. 'Skyscrapers' really highlights the clarity and detail of the bass.
Next I moved onto some of my favorite music; just about anything by Jack White. The White Stripes, by nature, were very simplistic but distortion heavy music. 'Im Slowly Turning Into You' off of 'Icky Thump' is a revealing song, where the drums and bass can interfere with the vocals at high volumes. This has been an issue on many speaker I have listened to, but not this pair. 'Instinct Blues' off of 'Get Behind Me Satan' has the same effect. With White's more complex music, such as that off of 'Blunderbuss', the speakers also shine.
With more simple acoustic-guitar-and-vocals type music, the speakers sound extremely lifelike, with voices being airier and natural, not so 'plasticy' like I have sometimes noticed. The guitar sounds like it's in the room with you, and not at all colored by the speakers.
So in conclusion: The speakers are very detailed and lifelike, and have extraordinary frequency extension. The highs and mids shout and are a bit harsh at high volumes,though this listening test was done with drivers that are not broken in. With the speakers costing my about 250 dollars for the pair, and being so simple to make, this was a great beginners project and a fantastic speaker design overall. Credit for the fantastic design has to go to Bjorn.

That was immediately after the speakers were completed. Now that they have broken in and the BSC circuit added, the sound has mellowed out and become much smoother, with the midrange shout and harshness gone. The bass has also become a little less constrained and more open. Very nice improvement!

Well, that's it! You have now completed your WiBAQs. Enjoy them and welcome to the world of DIY audio and full range driver magic.