A Built enclosure, complete with a sub and ports will resonate at a certain point. By using a signal generator on a laptop plugged into your amp running your sub you can test frequencies until you find at which it resonates at (will be the loudest point). You can trial and error tune for a set that you are looking for. It would be easier with round ports using PVC pipe of approprate dia. Was just an idea, how accurate would this be? Polo..
how long is the port (if it makes an L in the box, measure it right down the center)?
also...is your box 4.7cuft net or gross? (before or after port/sub displacements)
Polo's kinda talking about box resonance caused by port tuning and cabin gain. Depending on your box tuning and sub, you'll have an increase in spl at a certain frequency. If you model it with a program like WinISD, you'll see what I'm talking about. If you change the port tuning, you'll change the frequency that this occurs at. If you listen to a test tone cd, you will hear it, but it is not the same thing as what your port is tuned at. Now, your vehicle has its own frequency it resonates at. If you compete in spl, you try to get both frequencies the same, so you get a huge spike at one frequency. If you want SQ, you try to get rid of any spikes so all frequencies have the same loudness.