Whats a subwoofer?
Lets start step wish from top of the frequency band to bottom
Tweeter,
A small, usually some metal, hard in nature, driver, used to produce high frequencies, due to its small size, it can radiate really fast and requires less power.
Mid range
A medium sized cone, driver, used to produce middle frequencies, ( ~ 3-8inches)
Mid Base driver
A extended version of Midrange, having better low frequency response ( 6-8inches)
Woofer
A driver used to produce low frequencies. In 2 way systems, the word woofer is often used in place of mid base driver,
Subwoofer
A low frequency dedicated woofer, with its dedicated amplifier.
Enclosures
The BOX which houses the drivers, plays important role in the reproduction of sound characteristics, Since tweeter and midrange do not have much linear movement, less air mass been moved around, enclosures plays little to no role there. For woofer applications, enclosure design is VERY important. Below u will read some designs used for subwoofers/woofers.
Some copy pasting now
Sealed Woofer Enclosures
The simplest of all loudspeaker designs, the sealed enclosure system consists of a driver mounted on one side of a sealed box. The sealed enclosure system is characterised by excellent transient response, good low frequency power handling, smaller box size and lower sensitivity to misaligned parameters when compared to other alignments. However, sealed enclosure systems tend to suffer from higher cutoff points and lower sensitivity than the other low frequency systems.
They are usually the subwoofer system of choice for audiophiles because of their excellent transient response (i.e. no boomy sound) characteristics when designed and built properly. http://www.diysubwoofers.org/sld/
Ported Woofer Enclosures
A ported enclosure system consists of a driver mounted on one side of a box that has an open tunnel or port which allows the passage of air in and out of the box. At low frequencies, the vent contributes substantially to the output of the system.
The ported enclosure system is characterised by lower distortion and higher power handling in the system's operating range, and lower cutoff frequency than a sealed enclosure system using the same driver. Distortion rapidly increases below the cutoff frequency however as the driver becomes unloaded, and the transient response of a ported enclosure system is usually inferior to that of a sealed enclosure system using the same driver. However, the lower cutoff frequency and better power handling within the system's passband often makes ported systems the alignment of choice for many speaker builders.http://www.diysubwoofers.org/prt/
I strongly disagree with him here on the part that ported systems handle more power than sealed systems, ported are louder for sure, due to no air resistance, hence higher and easier cone displacement, this higher displacement when mated with higher power, can easily result in cone overshooting its XMAS, damaging its coil.
Band Pass enclosure
The 4th order or sealed rear chamber bandpass system is basically a sealed enclosure system with the addition of an acoustic filter in front of the driver. The resulting system usually provides a lower cutoff frequency, the tradeoff being a larger enclosure. The enclosure can be reduced in size by using two drivers in an isobaric configuration. http://www.diysubwoofers.org/bnd/4thorder.htm
This is NOT how most shop wala ppl make them, they usually make untuned ported boxes and call them band pass. This kind of enclosure requires most calculations and time. And is usually not used at personal level. We shall focus on other 2 more common enclosures mentioned abv.
Designing Sealed boxes
Designing these is easy, u need to know couple of theile/small parameters of ur woofer and ur done. Heres a list of what u need to know; Qts, Vas, Fs. Once u know the mentioned parameters, all serious manufactures give away all the details, next u need to decide the QTC for ur box, qtc of 0.707 gives perfect flat response,
Qtc values for designs that are commonly available are from 0.9-1.2. This means that most designs you have heard (unless you are listening at high-end audio retailers) are likely to have a peak in frequency response in the 70-120 Hz range. Many people like this and will be disappointed by the sound of a speaker without this "boost" in the bass.
Next u need to calculate Box volume, its easier to use web based calculators such as
http://www.carstereo.com/help2/Articles.cfm?id=26
F-3 means the frequency at which the box will reduce SPL by 3dB
Designing Ported Boxes
U need similar parameters for calculating ported box volumes, wts more important here is the port, length of port, diameter of port, and frequency tuned.
Use this calculator to find the required box volume, http://www.carstereo.com/help2/Articles.cfm?id=17
It suggests the port frequency, for flat response but its not always necessary to follow it :-D, u can always tune at frequencies u like more boost at, for me, it would be lowest possible frequency, once u have the idea about wt freq u want to tune it to, turn to port calculators, to find out length and diameter required to resonate air at that frequencies, some calculators allow u to select one dimension and calculates the other accordingly, always select maximum diameter u can, to reduce port noise.
Box details
When building a subwoofer enclosure you will find that
there are numerous materials recommended for construction.
1. Medium density fiberboard (MDF)
2. Marine grade plywood
3. Particle board (the smaller the particles the better)
These materials range in price as well as availability.
Particle board is the most common, least costly, and can be purchased at most hardware stores. MDF and marine grade plywood, although preferable, are usually much harder to find, and much more costly. Whichever you choose, we recommend that its minimum thickness be 3/4". This will provide the rigidity necessary for optimum performance.
For big boxes, always use some internal bracing to avoid wood resonance or boominess, to further reduce boomy base, one can use foam or polyester to absorb sound waves inside the box.
Some pics of my DIY
I and Guncha@PW tried to make this enclosure making thing make work, but unfortunately I was unable to cut the wood pieces precisely, hence was unable to seal the box, so decide I’ll let carpenter to his job, gave him exact dimensions we needed. Once done, got the box home, the sound to me was still boomy in sealed box, I added some white packing material in it, but no difference, than I added some foam, from a old cosion, its like 2 inches, on the base of box, well yea there was obvious difference insound, a bit lower in output, but lot clear. Got the woofer and amp in the trunk, at played some better quality mp3s/wavs I had. Hmm to my dispointment, my cars trunk is too live, it produces some echo, which results in boomy base inside the car, with direct listening the base is clear, inside the car, it gets boomy, I need people who are using woofers to suggest possible comfigurations which will reduce the boominess, inside the car, the woofer itself is tight, clear, and loud.
adding some absorbin material, this didnt help, so used faom for better sound proofing
running a frequency test to see how the subwoofer performs at diff frequencies, it had contant output from 90hz to 50 hz, lower at 100hz and 40hz, below 40hz, it was close to no ouput,
final product, box not made by me.
I will be updating this with recommend enclosure sizes for all popular woofers as soon as possible.