Ports

I am trying to build an enclosure for my new subs.  the parameters call for a 4 inch port that is 11.75 inches long.  the problem is that I have a 99 firebird and in the box I built that will pretty much hit the opposite wall.  someone told me that you could cut the port in two, so instead of having one port almost 12 inches long I could go with 2 ports about 6 inches long.  Will that work?  or will it mess up my tuning?

The consideration for port opening area is the factor called "port noise".  A two inch round opening can never be used for subwoofer use;  in fact, a 4" port is cutting it close for most of the subs available now.  When the port opening is too restrictive for the volume of sound, it issues noise that interferes with the box resonance sound.

You can follow the advice given above in changing it to a slot port, but bear in mind that the wall that separates the port from the air in the box is intruding on that air volume...which means that the box air volume got that much smaller.  A thin-type PVC tube will take up the least amount of box air displacement.  Buy some PVC and a 90 degree elbow.  Assemble the port so that it fits into the box with the 90 degree bend turning the port along the back wall.  Be sure to leave at least 3 to 4 inches of space, if measured straight out from the end of the port, to any partition, wall or the speaker itself.  Cut the port length with a pipe cutter after measuring through the center of the port tube from the front, or flanged end, of the port to the rear opening inside the box.  A steel flexible measuring tape will give accurate results here.  And remember that the given 4" diameter is inside dimensions, not the OD of the port tube.

If the tube you use differs at all from 4" ID, use the calculator and input the actual ID of your port to find the proper length to use to achieve the same tuning frequency.  It is also a good idea to re-measure and calculate the net air volume of the box at this time.