Grab a rubber band and a small masterlock (or any brand for that matter). Hook the band in the lock and hold it up in one hand by the band. Now slowly move the lock up and down using the band in your hand. Notice the lock moving at same freq and phase.
Now, pick up the pace a bit. As you move your hand faster, the lock starts to do something different. It's speed and phase starts changing. Now, go as fast as necessary to make the lock really bounce like mad. That is the resonant frequency, Fs or whatever you want to label it. Notice also how the lock goes 180 out of phase from your hand. Notice how little you have to move your hand to produce wide, rapid motion of the lock.
Now move your hand really, really fast. You see the lock has all but stopped moving entirely.

This is how a ported enclosure works. The lock is the weight of the air in the port. The rubber band is the springiness of the air in the box and represents the box size. Your hand is the speaker. The speed you move your hand is the freq of the note coming from the amp. How far you move your hand up and down is the power of the note wave coming from the amp.

Picture a box around the rubber band and picture a pvc tube around the space where the lock travels. At resonant freq, the lock comes OUT of the port into the car's air space and your hand comes out of the box into the car's air space at the same time. This is why it is so loud, you have 2 pressure fronts at the same exact time.

When you move slow, the lock comes out when the hand goes in. Now you have a + and - pressure front coming into the car at the same time. They largely cancel each other. That why there is not much sound from a ported box below resonance.

When you move very fast, the lock does nothing, acting like a sealed box, no pressure front at the port or a sealed box, either way, there is only the pressure made by the driver.

Try it for diff size bands and locks. Notice lighter locks with stiffer bands resonate fast. IE, small box with a short port. Notice a heavy lock with a wimpy band. IE, large box with long port containing lots of air. Didn't have to move too fast there, did you?

I hope this helps everyone in understanding enclosures.

PS, I have not been able to pin down a relationship between a spring constant 'k' and the way a speaker moves. Although, I'm sure some exist.

 

Once you've mastered that, I can illustrate why bandpasses are a pain.

Do the same setup as above, only this time, tie another band around your hand and secure it to a doorway or something so that your hand is straped from above. Now hang the lock down and try it. Alot harder to get it to resonate, isn't it. Gettin to be almost like a yo-yo, you gotta be good. Remember, you can't let the band go slack, that's cheatin and doesn't represent the real world, since air never goes 'slack'.

In case you don't know, the band tied to the door jamb is the air volume of the sealed chamber, the free lock band is the ported side's air vol. The lock is weight of air in port and so on.

Got that, now tie a another rubber band and heavier lock below the first set so you have a system on ports and boxes and only one speaker. Try this with and w/o having the upper one in the door. Guess what this one represents? Bose does alot of subs like that, don't know what to call it, but I love that design. You get 2 port resonances. I've been wanting to build one for a long time, just can't get the particulars worked out. Next time your at Sears or somewhere, peak in the hole of the bose sub for a home A/V setup.

No, I never tried this in water. I don't know what water would represent for sure. Any ideas? How about what gravity represents?