Port Tuning

It's not a matter of the volume of the port... It's a matter of the surface area of the oppening.

Basically, it works this way: The bigger the oppening, the longer the port must be in order to reach  a certain frequency, which is an inverse function. What this means, is that if you are going to double the number of ports, you either need to halve the port area, or double the port length, in order to keep the tunning frequency.

For a given opening size, the longer the port, the lower the tuning frequency.

If you think about it it makes sense... Lower frequencies have  a longer wave length, thus they must travel a greater distance ( port length) in order to come out in sync with the front wave.

Thanks guys!   That makes sense.  I'll check out the site you recommended kickerstang.

Here is a good calculator to use with figuring ports  http://www.carstereo.com/help2/Articles.cfm?id=31

I brought up your manual to have a look, and used the vent length calculator, knowing that the specs were for a 2" X 9" tube.  The tuning frequency came out to be 30.85 Hz. for your  SQ enclosure when built to the volume PG demands.  You should, of course, follow their recommendations to the letter.  Why would you want to use 2 ports?  If PG's engineers say to use one, I'd do what they say.

...(Just for general knowledge...a 30.85 wave length is 36.5 feet long, which is somewhat longer than the tube length! ...)       ;)

You see in the manual that the SPL size is 3 X 12.  That makes the SPL box tuned at 39.75 Hz.  Note that they prefer the SQ tuning for quality, but the SPL tuning "fits the basshead's need for maximum output  while sacrificing some sound quality".