Some question what enclosure is best. Hopefully this post will help people understand more about sealed and ported enclosures as well as bandpass.
Sealed is the smallest and the simplest of enclosures and will have good low frequency extension, but might NOT have the best low frequency. If space is limited, sealed would be your best choice. It's also the favorite type of enclosure for the SQ fans. (Sound Quality)
Ported will have better low frequency extension for a given response shape, but would require a larger enclosure. If you made the enclosure as small as the sealed, but ported it to gain low frequency response, the output would deviate from the desired flat response. They are also more difficult to build and need more space than sealed.
When using a sealed enclosure, it is difficult to be so far off as to risk damaging the subwoofer(s). In any type of enclosure the subwoofer relies on the enclosure for damping. If there is insufficient damping, the sub's cone will move significantly more than it should. The ported box will provide virtually no damping below port freq., all of the control will be provided by the sub's suspension. Below the tuning (port) freq., it is possible to cause damage to the sub well before power levels approach the maximum power ratings of the subwoofer.
Common Tuning Frequencies
Sound Quality(SQ): 20hz - 28hz
Sound Quality Loudness (SQL): 30hz - 35hz
Sound Pressure Level (SPL): 40hz - 80hz (Test tones are played rather than normal listening music, to avoid low freq., which damages the speaker. It's commonly used for competition purpose only and not daily driving. Unless you like listening to test tones for your everyday driving music)
Bandpass enclosures can sound good and give you a flat response, but most bandpass enclosures (prefab) are not designed for a flat response. This means that they are built to produce a large peak at some freq. from 50hz - 60hz. These enclosures work well with rap, but they don't sound good with other types of music. If you use a bandpass enclosure it should be specifically designed for your sub(s).
Calculations & Formulas
dV = is the required diameter of port in inches
Fb = is the tunning frequency of the enclosure in hertz
Vd = is the volume displaced by the driver (woofer)
Lv = is the LENGHT of port in inches
R = is the internal radius of vent tube
Vb = is the internal volume of the enclosure cubic inches
To convert cubic feet into cubic inches
# cuft *1728 = value in cubic inches
Vd = Sd x Xmax
Lv = [(1.463*10^7R^2)/(Fb^2*Vb)] - 1.463
Value of R
R = square root (a/3.142)
and "a" is the area of square vent = Width*Hight
changing Round vent to Square vent
A= Pi* (4/2)^2
and Pi =3.142