*OFFICIAL* Differences in Subwoofer Box Types

[CrEaTrE]

Now we will start with :

Sealed enclosures: A SEALED enclosure will be the smallest,and is the simplest of enclosures and will have good low frequency extension, but might not have the best low frequency extension. If space is limited.
and its a good choice for SQ fans (Sound Quality).

PORTED enclosures (Bass Reflex) : A PORTED enclosure will generally have a 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 enclosure, but ported it to gain the low frequency response, the output would deviate from the desired flat response.

Ported enclosures are slightly more difficult to design than a sealed enclosure. When using a sealed enclosure, it is difficult to be so far off as to risk damaging the woofers. In any type of enclosure, the woofer relies on the enclosure for damping. If there is insufficient damping, the woofer's cone will move significantly more than it should. The ported box will provide virtually no damping below the port frequency, all of the control will be provided by the woofer's suspension. Below the tuning (port) frequency, it is possible to cause damage to the woofer well before power levels approach the maximum power ratings of the woofer.

common tunning frequencies :
Sound Quality : 20 up to 28 Hz.
Sound Quality Loudness : 30 up to 35 Hz
Sound Pressure LeveL SPL : 40 up to 80 Hz (test tones are played rather than normal music, to avoid Low frequencies, which damages the speaker).
Its commonly used for competition propose. and not everyday driving.(unless you like listening to test tones for everyday driving).

BandPass enclosures (Single Reflex): Bandpass enclosure can sound good and give you a flat response,but most of the bandpass enclosures(in the market), are not designed for a flat response, this means that they are built to produce a large peak at some frequency from 50 to 60 Hz, these enclosures will work well with Rap, but generally they don't sound good with other types of music. If you use a bandpass enclosure, it should be designed specifically for your speaker.

P.S : its not always recommended.





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.

PS : to convert cuft into cubic inches

# cuft *1728 = value in cubic inches

Vd = Sd x Xmax

dV>/= 39.37*(Fb*Vd)^0.5

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

Source: SEALED vs PORTED (basic info.) - Car Audio Forum - CarAudio.com