So I've got two mostly identical pairs of bookshelf speakers. One pair was built from a kit that included a prebuilt cabinet, the other has the same xovers/mid/tweeter but I built the cabinet myself. The prebuilt boxes have a port that is 2 1/4 inches inner diameter by 6 inches long. The ones I made just have a hole, I never finished putting the ports in.
I finally got port tubes, but they are 2 1/2 inches instead of the 2 1/4 inches needed (they are longer however, I'll cut them to the size I need). My question is, having long since lost T/S parameters, box plans, model numbers, etc - is there an easy way to figure out how that extra 1/4 inch of inner diameter should affect the length of the port ? The ideal is 2 1/4 by 6, so should 2 1/2 be a little longer, a little smaller ?
You're looking to maintain the same total volume in the port. You can cut the larger diameter tubes a bit shorter (pull out your calculator and plug in numbers until you arrive at matching volumes) but cutting them to the specified length probably isn't going to make any noticeable change in performance.