One of the most important  parts of any speaker project is selection of the  speaker driver.If you're building a kit or working from a set of plans, in most cases this has already been done for you. Still, even in the case of a kit, many different drivers can fit into the same panel cutout and you may wish to select alternatives. If you're building from scratch, driver selection is a required part of the design process.   

Consider what is the primary purpose of your speakers first.  Is your application for a home theater (HT) system, or for music? Most people will use theirs for both, so here are the basic guidelines you need to observe:speakers

Home-theatre  multichannel requires at least 6 speakers  (some 8 Speakers)  two main (left and right front) speakers a center (front) channel speakers  two rear channel speakers and a subwoofer. Digital Dolby can require still more speakers. For HT systems  the recording engineers are in control of the sound field, so all 5 of the main and rear speakers should sound pretty much alike. Also, since there are so many of them, they should be as small and cost effective as possible. If you don't want to make all 5 alike, you should at least try to make the front 3 alike and the back 2 alike, both sets with a common "sound". The front 3 should also be magnetically shielded to avoid interference with the TV screen. All should have response down to around 80 Hz, where the subwoofer takes over . Since one of the things that contributes to the realism of HT is the bass, don't scrimp on the subwoofer!

DVD  movie sound tracks and DVD audio discs offer multiple channels but ,most of your attention should be paid to the quality of sound coming from your front two speakers and the centre channel as they  provide the location and depth of field for the music or video  effects .The centre speaker is the one of the most  vital links in  multichannel digital surround systems providing the left to right movement of people  effects objects and instruments steering you to the correct location in the sound field  . If you wish to use a subwoofer, you will either need an electronic crossover in the signal path used in conjunction with a separate power amp , or  the preferred easy option and less involved is to purchase a subwoofer amplifier which has the necessary bass filters and phase switching built in already  and can be mounted  straight into a subwoofer enclosure ready to go  . 

 Shielded  drivers The two front main speakers may be located far enough away from the TV screen to not necessarily require shielding but the center channel speaker will require shielding. Rear channel speakers in such a system should share the same sound as the front channel speakers  usually using similar drivers with the same cone material and sensitivity 

Choosing the number of drivers and their specific parameters sounds easy but just remember, the more drivers you use, the easier it is to mess things up and the more complex the hole design becomes . Two-way designs are the simplest and can sound amazingly good with high definition  drivers . Three-way designs require more care to properly match all the speaker drivers in sound character (cone type) and SPL  level . Four-way or higher-order designs get really complicated. Often, these will have crossovers in the middle of the critical musical range of 300-3,000 Hz, which makes driver matching much more critical and is very time consuming.

Choosing  the type of enclosure for your driver using Thiel and Small parameters as a starting point Use a computer speaker design program like one of those listed on the contents page and apply these simple rules 
A very  basic general rule of thumb for enclosure types  which will allow a you to quickly choose a driver or several  drivers for your preferred type of enclosure and quick calculation of size 

Ots  < 0.5  use a vented  enclosure
Ots  > 0.5  use a sealed  enclosure 
Vas  with values (  5 to 60  ) litres  will result in a small to medium  enclosure size
Vas  with values (80  to 250) litres  will result in a  large enclosure size

as a general rule of thumb the vent size should be around 25% to 40% of the speakers piston area 

Sealed enclosures
do not have the bass extension generally as a similar sized vented enclosure but can be placed against or very near to walls in corners or in wall  cabinets which can supply a  boost in the bass level between 4 to 20db through room mode gain and compensate for the lack of bass. Sealed speakers do not suffer from vent velocity turbulence or wind noise of the vented enclosure  but drivers usually require a larger Cms (compliance)  a higher power rating and a lower Fs resonant frequency .The box or enclosure is usually lined of filled with acoustic damping material to reduce excessive cone movement and fine tune the system Q of the box ,remembering that the speaker acts like a spring that has to expand and compress air inside the chamber as the cone moves in and out with the music signal .Any poor glue joints on the cabinet will result in air leaks and thus poor and distorted sound so be liberal with the glue when building the cabinet .Sealed enclosures can be used for 4 to 6' drivers as a midrange cabinet where very low frequencies are not required and thus  drivers with  smaller cone excursions can be used .  Drivers  normally requiring a vented enclosure can be placed into a sealed box and used as a midrange for a 3 way speaker system where the low frequency roll of the cabinet driver combination can assist the demand on the crossover 

Vented enclosures have their advantages over the sealed enclosure with reduced cone excursion at and around the resonant frequency of the System ( driver ,speaker vent and enclosure ) also providing an extended low-end response compared to a sealed enclosure of similar size .Increased  power handling of the driver at low frequencies around the system resonance  is improved and greater flex ability of system performance. Upgrading can be performed by only changing the vent length and diameter rather than changing the complete cabinet size as is required with a sealed enclosure. Small changes in the bass response can be achieved by varying the amount thickness and density of the acoustic material lining the inside wall or walls of the cabinet. Vented enclosures are tuned to get the best performance from the driver by the addition of a vent either in the form of a  PVC  tubular round pipe  or rectangular vent  made of the timber, as a general rule of thumb the vent size should be around 25% to 40% of the speakers piston area .Thiel and Small (two Australian Engineers  from Sydney ) developed the world  Speaker industry standard of  acoustic speaker parameters  when they derived a series of mathematical  filter functions to calculate the enclosure and  vent and size from a set of parameters measured from the speaker driver itself, these parameters are usually entered into a computer software program for speaker box design .These Thiel and Small (T&S) parameters  are available  from the manufactures  or for higher accuracy SSS LEAP (T&S parameters ) see those provided in this book or on line. 

Bandpass enclosures are simply a high pass enclosure (vented ) with an acoustic low pass filter (chamber) attached to the front of the vented enclosure which gives a flattened mountain top or bell shaped frequency response .The  SPL (speaker pressure level) and efficiency is greater than a vented enclosure because of the reduced power bandwidth . Can be used as a  subwoofers when no active filters or subwoofer amp is unavailable , Is used in the car and PA  industry where high efficiency is on demand .Not  normally recommended for home theatre or hi end sound ,cabinet construction can be  complex   

 Subwoofer Enclosures
are usually  vented or sealed depending on the type of driver .Vented is the most common type used through the Home- theatre industry because of the size and efficiency obtainable from today's high powered high  efficiency drivers. They can be vented any where on the cabinet and even with the vent facing the floor providing you allow at least 50mm spacing above the floor for the vent to work. At very low frequencies a developed in DVD thx movies the subwoofer driver must have response down to at least 30hz and down to 20hz for the larger 12 to 15" drivers . The inside walls of the cabinet  need to be very well braced  with timber allowing  at least 25mm thick MDF (medium density fiber board ) front side  and rear panels so as to reduce any baffle resonance to a minimum . The frequency response bandwidth is between 20 to 180 hz so a good long throw low fs high powered subwoofer driver is a must. A subwoofer is  highly recommended for Home theatre  speaker  systems and the size will depend on the size of your speakers and room .  I recommend Subwoofers  6" to 8" for small to medium rooms an 10" to 12" for large rooms  15" for extra large rooms 

Choosing the right type of  driver for your system 
The type of driver will depend on the sound program material and type of system 

Digital Home-theatre systems 
Stereo  music systems 
PA  music systems 
Intercom conference  system
Multi-room  AV  domestic house system 
Computer DVD and music
Internet in room monitors

Digital Home-theatre systems speaker drivers  need to be matched  as near as possible in sound signature ie cone  material type power rating and SPL sensitivity levels so as to obtain similar sound signatures  from each of the surround channel speaker enclosures.
I  suggest  that the front speakers are probably the most important and if you are on a budget apply most of it to the fronts . 
Digital HT and Music Speaker Bandwidth The speaker system  needs to have a wide bandwidth  to reproduce the demanding and dynamic sound tracks from a movies and music.
I recommend choosing drivers with wide bandwidths with a flat frequency response and 
a smooth roll of at the upper and lower frequency extremes .Metal dome tweeters or metal cone bass-mid drivers are generally  harder to apply when designing a speaker system with  rivers usually require more complex crossovers due the inherent cone breakup modes
but can sound good when properly applied .
The speaker system for Home-theatre  movie and music should have a frequency response  from a minimum of 60hz to 20Khz preferably more in the top end to 40Khz if possible with an active subwoofer unit covering the lower octaves from20Hzto180Hz Remember that as technology improves so will the demand for wider bandwith  systems so look always to the future when building a speaker sys tem. 

The sound quality of the multi-channel movies or music sound tracks depend on what studio the DVD or Digital recording  was recorded in eg (Sony WEA Artisan Entertainment CBS 20th Centry Fox ) and  who did the sound mixing this puts a big variable into  the sound quality which can vary from being  very dynamic  to dead flat undynamic unexciting  this is why some DVDs sound better than others . If your speakers are properly designed 
and are very detailed and dynamic you will be able to hear the radical differences in sound quality and realy  make listening an enjoyable experience so do not  scrimp on the cost of the speakers they are usually a long term investment