port length, etc.

Use math to convert round to slotted (following is a reprint of a past reply):

pi r^2 to find the area of a circle

pi is 3.14

You want to change the shape of the opening but keep it the same square inches.  The length of the port is not a factor and you will leave it exactly the length it already is.

• Find the square inches of your circle using pi r^2 (ex. a 6 inch diameter circle yields 28.26 square inches)
• Decide on the longest side you want your square or slotted port opening to be
• Divide the total square inches of the circle by that long side, in inches (ex. 28.26 / 11.5 = 2.46)
  The result is the measurement of the shorter side of the port opening

In the example, the 6" round port equals a slotted port 11.5 X 2.46 inches.  Round measurements to the nearest Imperial or metric value.  2.46 would be  2 7/16 to 2 1/2.

You should keep the ratio of length to width of the slotted opening at no greater than 8:1.

I don't know how to take the information that I see on that Soundsplinter site.  With the very small box size recommendations, port length is astronomical...certainly not feasable at 8 feet long.  Yet that is what they claim is the optimal car response:  .45 cu ft tuned to 27 Hz.  Even doubling box size out of the "optimal" size but still in "acceptable" range at 1 cu ft, port length with only 12.56 square inches of opening (4" port, which I would consider minimal) is still a very long 43.5 ".  Generally, we see much larger enclosures for vented boxes and thus shorter port lengths.

I edited this to add:  I took a look at this pair of subs in a .9 cu ft net airspace box using WinISD.  That's going with the optimal recommendation set forth on the site, and tuned to 27 Hz.  It's a nice rolloff that looks a lot like a sealed box response.  There is no problem with port noise using just one 4" port for the two-woofer box.  And there is no problem with cone excursion.  I gave the program a maximum 1000 watts for the two subs and I don't see a problem with either one.  It looks like a good sounding sub system (see yellow response line below).

soundsplinter_i_10.jpg

Port length is long, but using only 12.56 square inches of opening for the entire box brings the length down to 48.5 inches.  You may want to either use a round port or build a square port and incorporate that into the box.  The length of the port doesn't work in a typical situation that you might see where the port just goes back to the rear of the box and turns a little bit.  This port will turn a few times to get that 4' length into the box.  Keep this in mind, though:  you will save a great amount of space by using PVC round port instead of building the port from box MDF material, with this much length to deal with.  It's a small box if you think only in terms of net airspace, but a much bigger box when the port is included within.

Of course, be sure to deduct port volume as well as woofer displacement from total box airspace to arrive at net airspace.