Hey ZeonRider, I won't claim to be the most knowledgable about reading driver datasheets, but I have done some reading in a couple of books. I will look through the one I have at home and see if I can come up with some more information for you.
Hello Again ZeonRider, At long last I am getting back to you. Sorry about the delay. I don't always claim to be the most prompt. If this is your first speaker project, I would suggest keeping it simple and build a acoustic suspension (closed box) cabinet. The advantage is that it is much easier to make changes like adding ports, etc. than it is to remove them. The three parameters of the driver that are most important for this are:
Fs - free air resonance of the driver
Qts - the Q factor of the driver
Vas - the compliance of the driver, stated as equivalent volume of air.
Here's formulas to help the design process:
alpha = (Qtc/Qts)^2-1
Qtc = system Q of the box and driver, Qts = see above
Vb = Vas/alpha
Vb = box volume, Vas = see above
The Q of the system determines the type of bass you should obtain.
A Qtc of 0.7 will provide the flattest response. If you make Qtc smaller (i.e. 0.5) then the bass will go lower in freq., but it won't seem as punchy. If you make the Qtc higher (i.e. 1.0, 1.5, etc.) the low bass won't be there, but the midbass will have more punch. This is more of a personal choice.
To determine the theoretical resonant frequency of the system:
Fc = (Qtc/Qts) * Fs
Looking at the datasheet for the 21W54
Fs = 30Hz
Qts = 0,303 (0.303 here in the USA)
Vas = 59,6 liters (59.6 liter in the states)
A rule of thumb for box volume is for an 8"(200mm) driver is to make a box with a volume between 0.6 and 0.8 cubic feet (17 to 23 ltrs).
If you are good at woodworking, making a box with a sloped front, so that the center point of the voice coil of the bass driver and the center point of the voice coil of the tweeter you decide to use will help with time alignment of the two drivers.
I would suggest having separate terminals on the back of the cabinet for the woofer and tweeter, and making the crossover external. This allows you to experiment with the crossover design to optimize it to your taste, or flattest response.
I hope this helps a little bit for you. I am sure that there are others here with much more experience than I in this field, and many will have different opinions. I don't say that this is perhaps the best design for this driver, just that starting with a closed box is probably the easiest.
Good luck and please post pics of the finished project. Subjective comments about its sound would be cool also. And if you are really lucky, some actual test data would be nice also.