Why do you need an enclosure at all?
Picture a sub moving. The cone moves in and out, rapidly - and it's that simple.
Picture the cone moving out, in slow motion. It compresses the air in front of the cone, and rarefies the air behind the cone.
IF the sub were just sitting there, the compressed air would largely simply bleed into the rarefied vacuum behind the cone, and most of the sound would simply cancel out as a result... leaving you with not much sound at all, if any.
Also a good time to mention Hoffman's Iron Law:
You can only have two of the following three at any one time;
1) Small Box
2) High Efficiency
3) Low frequency extension
If you want any two of them, then you must sacrifice the third.
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). The sealed box simply contains the rear-of-cone energy, so that it can't escape, to cause the cancellations with the front-of-cone energy. The front-of-cone energy is what you ultimately end up hearing.
The sealed box also, inherently, ends up affecting cone motion - because the trapped air inside the box acts as a spring, essentially. It's more difficult for the cone to move in and out in an enclosure, because the air is preventing the cone from moving in and out, it's resisting being squished and stretched.
As a result, if you put a sub in a tiny enclosure, it inherently will always be inefficient - requiring lots and lots of power to even get moderately loud - because you are trying to brute-force your cone motion... a large enclosure is much more efficient, requiring less power to get as loud.
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).
In this case, a vent is installed in the enclosure. Might be round, might be rectangular, might be any shape... ultimately it doesn't matter - what does matter, is that the port opening area and length of the port have to be sized such that they provide just a little resistance against air passing through it. The amount of resistance is critical to the performance of the enclosure.
In this case, essentially you have the same properties of the sealed enclosure, but the vent does something interesting...
Again, picture the sub in slow motion.
Picture it moving in, compressing the air behind the cone, inside the enclosure.
Naturally, this air tries to escape out the port, having suddenly been pressurized.
...and it does, but the port slows it's escape. As the air is travelling out the port, the sub has changed direction, and is now heading out again.
By now, the compressed air has travelled out the port - just in time for the cone to move out again - and now we have a situation where the front-of-cone energy is creating a pressure node in the listening space, and the rear-of-cone energy is creating a pressure node in the listening space, thanks to our friend, the port.
...and continuing the thought, the cone has moved out, rarefying the air behind the cone, creating a vacuum effect inside the enclosure, sucking air in the port... by the time the air can be sucked in through the slightly resistive port, creating a vacuum outside, the cone has again moved in, creating a vacuum node itself.
One common misconception is that since it has a hole, the pressures must be lower inside a ported box, compared to a sealed box.
The opposite is true, however!
Again, thinking about what is happening in slow motion - with the sealed box, you simply have the sub compressing and decompressing the air trapped inside the enclosure.
With a ported box, however, you have not only the sub, but the momentum of the air in the port pressurizing and depressurizing the air inside the enclosure.
As such, pressures are actually HIGHER inside a ported enclosure than a sealed one.
Things aren't perfect in "ported land" however.
The port is a static thing, and as such, it's air resistance - and therefore it's delay, are a static value.
Wavelengths, however, are different for every frequency, increasing in length as the frequencies decrease. As a result, one wave takes more time to pass for a low frequency, than for a high frequency.
As such, you end up building a port that's "ideal" at one particular frequency - but as you move away from that frequency, you either overcompensate for the frequency being played, or undercompensate for it - but either way, the phase relationship is degraded, and you again move in the direction of being more or less like a sub that isn't even in an enclosure at all.
...In fact, if you operate a subwoofer too far below its tuning frequency, the air moving in the port is cycling too slow to increase, or even equal the pressures inside the enclosure - and it acts like a simple hole - resulting in little or no air 'spring' value inside the enclosure - making it very easy to bottom out your sub with even less than the RMS powerhandling limit of the sub being sent to it.
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 dont sound good with other types of music.
if u use a banpass enclosure. it should be designed specificaly for your speaker.
P.S : its not always recommended.
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
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
Edited by the727kid, 29 November 2007 - 09:39 AM.