Mathematical models of loudspeaker designs all make assumptions or are based on some ideal situation. Some parameters like box losses (Ql) can only be determined after construction. Whether free or expensive the software programme or person using it has to estimate (guess) the value. If you read through enough posts here or on other forum you will come across people advising making allowances or tweaks based on past experience. Sometimes the best looking response doesn’t sound the best. If the theory gets you close enough then don’t sweat the details. One advantage of round ports, of the plastic type, is that you can make them a tight fit and longer than you think and then measure and adjust before finally fixing them in position.

Back to your question. All ports or vents have some resistance caused by the air in contact with the port wall. Multiple ports have a bigger surface area than a single port with an equivalent area. The resistance of air flowing over a surface varies depending on the speed so in a port it will vary depending on the audio level as well. The same thing applies to a narrow slot type port as with multiple ports. The more you move from square the bigger the ratio of surface area to volume. The larger surface area increases the resistance and lowers the Q. 

endre_K, can you keep similar questions to one or two threads at the most. For old codgers like me it gets a bit confusing with all the similar threads that you have started. So the following is in response to your multiple drivers question.

Using more than one drive unit changes the Vas. If the enclosure size is constant then yes fs does change. It is normal to put multiple drive units in a proportionally larger enclosure to keep the response similar to the individual drive units. The isobaric technique uses two drive units in the same cabinet to improve bass performance for a given cabinet size, but that is at the expense of efficiency. I've only seen that technique used a few times for PA work.

If you want to learn the underlying theory you need to read some books on the subject, which will give you a more complete picture of what is going on.

The two standard texts which cover a lot of theory are:

 ‘Acoustics’ by Beranek and ‘Acoustical Engineering’ by Olson.

 ‘Dynamical analogies to engineering problems’ also by Olson is useful if you have any electrical engineering knowledge. (pop quiz question 1: what's the rotational mechanical analogy of two resisters in parallel?)

‘Bullock on boxes’ goes through some of the original alignments.

Google should bring up a lot of information about the Thiele Small papers.

More tuned pipes and horns Martin King’s website has a lot of information.

Re: the question about lowering the Q of the port. This is considered a bad idea because it increases cone excursion. There was a trend in the 1980s of adding straws to ports to improve the laminar flow through the port to reduce noise.  This too had the side effect of adding resistance and therefore reducing the Q. For multiple ports, or slot shaped ports I wouldn’t think that it makes that much difference.

I’ve tended to keep to relatively conventional designs so not looked into the problem at great depth, nor have I come across much theoretical work which discusses it (that’s not to say it doesn’t exist). Most papers on port noise tend to concentrate on flaring the ends of the port.

Steve_B, sorry about it, but i need to finish this school project for  june, and i'm a bit in a hurry, and nobody answered...i got stressed. Sorry again won't happend anymore.  :(

June is ages away. 

I tend to follow threads that I have contributed to and was puzzled when I couldn’t see any of my posts in one of the other threads thinking it was this one. As I said I’m not as young and sharp as I once was.

If you wire two drive units in parallel in one cabinet double the volume that you would use for a single drive unit and then tune the port to the same frequency as you would for a single drive unit. In the formula that you posted, the value of Vb doubles and Fb stays the same. The other variables can be what you want. If you alter the diameter (area) of the port, that will affect the length.