everyone says they have this killer setup in a slot ported box yada yada....tuned to 35-40 or whatever hz. but how do you tune a box to a certain frequency and how do you know what frequency to tune it to?
Your subwoofer T/S parameters should state the recomended size ported enclosure and the optimal frequency for the sub. If one is not listed call the manufactur; they may tell you the sub does not work well in ported enclosures.
Once however you have the size and frequency theres a formula (port length calculator) that tells you how long to make your port. The length and diameter of the port is what determines the frequency the box is tuned to.