ported boxes want to learn

Start with this:  what program are you referring to?

And I'll add some to this.  For the first question, a vented driver needs a subsonic filter, which is actually a highpass.  You have to set the crossover on the amp on lowpass so that you can crossover to the mid drivers, but the additional subsonic filter will control the cone movement below the box tuning frequency. Some amps have a subsonic filter available.  A sealed enclosure doesn't need one.

For #2, here's a quote from http://www.diysubwoofers.org/prt/ported1.htm that you may have already read: 

Note that this is only one of many possible alignments for a ported system with a given driver. Many types of alignments exist, each with its own particular advantages and disadvantages. This particular alignment will give you a maximally flat response within the system's passband, but it won't necessarily produce the best results with every driver, particularly those with very low Qts values.

This is in reference to the formulas you referred to and how the formula will seek a response.  As you can see from the quote, that's just a starting point which you can change as necessary.

When you use a design program you can quickly find different responses by changing the Fb or tuning frequency.  You'll see that the F3 changes with every change you make.  It is entirely up to you as to where you want the tuning freq.  From my very limited understanding of F3, it is a reference point derived from the output level in db of the driver in your designed box.  That reference is 3 db down from the normal db level, not from the peaked level.  It gives you a reference point where you would work in a crossover, or with a sub, a subsonic filter.  I'm sure there's more to it but that's how I look at it.

And because the F3 changes with every change you make in tuning frequency, it is obviously not a boundary of any sort...just a reference point for setting crossovers.

Here is a description of the Unibox modeling program that you will find useful.  And some more links to info:

http://www.diysubwoofers.org/faq.htm

http://home20.inet.tele.dk/kou/ubmodel.html

http://www.lalena.com/audio/basic/terms/

http://www.hometheaterhifi.com/volume_5_2/cmilleressayporting.html

http://www.carstereo.com/help2/Articles.cfm

http://www.clubknowledge.com/Car_Audio_FAQ/?t29

http://www.linearteam.dk/default.aspx?pageid=winisdpro

http://www.mhsoft.nl/spk_calc.asp#top

Build the box so that it performs well in the worst case scenario and, in return, it will reward you at all times.

OK so i got it now i the amp is on low pass which i figured. And im not building a box just want to learn about it more so when i do i can make a good one i was using the calculator on this site and the program WinISD. Ya i was up a while last night reading and figuring out more of this program so hopefully maybe you can answer me these quick questions the F3 is the point where if you want to tune your box below that mark it wont work properyl right? and when your tuning your box with that program it is generally better to try and get a smoother wave correct and the drivers i was practicing were some JLaudios like the jlw6 2 12 incher found on thier site. And also when your tuning your box is it better to try and get it as low as possible (frequency) and this number how is it incorporated when you hook it up like for say i made a ported box with jl audios w 6 12 inch  the box is tuned to 27 hz (you can check it if you want) www.jlaudio.com find 12 w6-2 info if you have the program i tuned it to 27 or so a box of 4.6 cubic feet, retangle port of  4 by 6. you can see the results ive compentated for the area of the port in the box you get a box of like 21 by 33 by 13 inches used 19mm thick wood all included. maybe try it see what you think if i did it ok or not. That driver isnt included with the program so i added it  form website here ill give you what i used Qts=0.45 VAS= 2.82 cubic feet   Fs= 25 Hz Xmax=16.5mm Qms=7.1 Qes=0.48 and Pe= 400 watts (Rms)

ok i dont really have time to go through all the links right now but if you can message me with how to make a subsonic filter or a link with how to understand it that would help .

Ok forgot a few things and hopefully learned a few things reading on subsonics forgot to mention the f3 in that equation above it came out to 19.9Hz and the Fb came to 21.54Hz originally but i modified the tuning frequency in the program to get a workable port length (was too long at that frequency). ok the amps usually come with crossover  so if theorectically i made this box with that sub  and connected it to the amp i would set the amp on LP i guess amps start at higher frequencies like 35-40 Hz or if possible get it closer to the FB i used to tune my box 27-30 right (usually see on amps crossover begins at like 35Hz). and if i got subsonic filter on the amp i would set it to 20 Hz to prevent it from going under my f3 point right? if you could answer me this please

I found the time to post them for you though.  And I believe you DO have the time.

Those are just a beginning.  With some diligence, links from this site and others, and Google searches you can find all the information you want or need.  Read through the entire help section of WinISD.  It speaks volumes.

It's not uncommon to spend more than a year learning about designing vented boxes.  A sure way to speed up that learning curve is to get started on a project, whether it be for car or home audio.  It's like OJT, and everyone knows you learn faster on the job than with your nose in a book.

And please....use punctuation in your posts.  Capital letters, periods, commas, sentences, paragraphs....  Major headache time trying to work our way through your writing. 

AHAHAHAH ya i do, but the thing is i've read all that already im just trying to ask you questions to see if i understand what i read. Thanks, some helped didnt know about subsonic filters and stuff. I just have a few things to clear up here. First F3 this frequency  is the frequency that you cant tune your box under and it should be the frequency you set your subsonic filter to right? Secondly if i tune my box to 27 hz for example will it sound good when i play with the crossover frequency on my amp? (most amps i see crossover frequency begins at 35Hz) or is it better to tune the box higher than that,  buy a crossover, or make one out of caps and coils?

Sorry about the grammar been out of school a while.

A definition of F3 is that it is the -3db point where the output power has fallen to one half and so is considered to be the low frequency limit of the loudspeaker. ( http://www.highefficiencyloudspeakers.com/user/Alignments.pdf) 

When you put a speaker into a vented enclosure you know you are (or at least you CAN be) extending the low freq response by using a port.  So, knowing that, you know that if you change the tuning frequency by manipulation of the box size or port size, the F3 will change along with it.  Keep the Fb above the driver Fs when finding a tuning freq.

You probably shouldn't be worrying about F3 so much.  In that link above they show 3 formulas to use with a driver (of your choice) to see if it will suit your application.  By using the formulas, you will find F3.  If it is too high for what you want, choose a different driver and start over.  You can use the T/S parameters given for each one and choose your speaker that way.  Or you can use a nifty program like WinISD Pro or Unibox and let them do these calculations for you, like I do.

If you try to manipulate the low end (F3) of an enclosure too far you'll get problems in other areas.  As you work with different drivers you'll see there is a limited range of changes you can make and still have a good sounding loudspeaker.

As far as a subsonic filter goes, you just want to make sure it is set just slightly below Fb so that you still maintain the full complement of the port but the filter controls the cone movement at freqs below the tuning freq.  Which leads to your second Q:  it is often desirable to tune an enclosure below the point at which you will start crossing it over or filtering it out.  You don't get the full effect of the port at Fb because it is crossed out, but you can sometimes extend the low freq response by doing it this way instead of using a sealed box.  And drivers are, you know, made to work better one way or the other, although there are a lot that can go either vented or sealed.  As you will read over and over, it depends on what you want to accomplish, what your working with, the driver, etc.