sub box

You shouldn't have to rework the figures 100 times.  Just 40 - 50 times should be sufficient. 

Yep!  That's why you model in software before you cut wood.  Making 40 or 50 or 500 changes in software is as easy as a click of the mouse.  Experimenting and discovering is part of the fun! 

10% the speed of sound or 110 fps is pretty high, but usually acceptable in car audio subwoofers.  I suggest it's too high if the port is in your listening space (like in an SUV)  and I suggest you design for a max velocity of 5% of the speed of sound, or about 55 fps.  As far as what size and shape to use for the port, that is completely dependent on the size of the enclosure and the amount of power used.  Post those if you can.

Also, Raven's suggested tuning is good for car audio.  What is the fs of your drivers?

My answer was tongue-in-cheek, of course, but you ask a very good question.  It is a major stumbling block.

The dilemna we're referring to is about the need to change the net cubic feet of the enclosure when the volume of the port changes.  For each tweak on either side, a corresponding change in the other side of the equation creates the need to affirm the design all over again in order to maintain the same tuning frequency (with further fiddling and tweaking of tuning freq applied along the way, as well!).  We could almost say that the tweaking could go on infinitely.

A method of making this easier (and of paramount importance, which you are doing) is to use software to work the design.

• Open up several instances of the woofer in the program, all at the same time.  Use topologies that differ from one another:  a couple of rectangle ports, some different sizes of round ports, big differences in net cubic feet....
• Use the port air speed chart to weed out port openings that appear to be too small.
• Have a few good calculators open to work with while you are working with the box building program.  One is the simple calculator that comes with every PC.  I often have up to five or more instances of just that calculator open at various times.

Continue the process of elimination until you weed the field down to the best one or two designs.  At that point you should be able to fine tune the airspaces with just a few tweaks.

Here are some links to save:

http://www.madisound.com/services/hifispeakerdesign.php

http://www./help2/Articles.cfm?id=31

http://www.ajdesigner.com/phpsubwoofervented/efficiency_bandwidth_product_equation.php

http://www.1728.com/pythgorn.htm (for angled box building)

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

http://www.joshmadison.com/software