The basic sealed cab has a very high single resonant peak, which means that when you hit it, the box could have quite a dead spot. The purpose of tuning is to reduce that peak. It's a compromise because you end up with yo smaller peaks. This problem and cure was discovered by Helm Holtz from which the port takes its name Helm Holtz Resonator.
My own method of tuning before computers came about was to use a volt meter connected to the output of an Amp. An audio generator signal was fed into it and the needle would move twice in 2 different places. This represented 2 peaks. I had adjustable ports and altered them until the peaks were level. At that point the cab would be correct.
This only made the box technically correct. It didn't mean that you got thumping bass out of it. Todays method with things like Bass Box, you are able to drag down the bass as far as you want to an acceptable level usually meaning that the 2 impedance peaks won't be level. However, it's the sound that counts.
Before my original method there was the ping pong way. You would alter the ports until you got ping ping instead of ping pong when you listened to the sound from the port, and then you'd get in the old tin bath for a good soak in front of the fire.
Peace and goodwill to all speaker builders
