WARNING: BORING SCIENCE AHEAD!!
It should allow the back wave to travel freely to the outside environment to add to the front wave. A ported enclosure is what is known as a Helmholtz resonator. What a HR is is a cavity of air with a neck or channel at one end. When the pressure inside the cavity is increased, the air wants to go to the area of lower pressure, aka, the outside, so that it is in equilibrium. Now, think about your speaker cone as a spring: as it moves in, it compresses the air in the box which increases the air pressure inside the box compared to the outside.
Unfortunately, when the compressed air inside the cavity encounters the internal mouth of the port, the air will have momentum to it and the cavity will consequently overcompensate for this and push out too much air. Why does it have momentum? The air in the cavity is of a much greater volume than the air in the port and by fluid dynamics, to account for the difference in pressure, the air in the narrower channel must move faster than the air in the box is actually being pressurized. That's also why smaller ports have higher port velocity: the air in the port has higher momentum which as you may know is a function of velocity, or as we are concerned, acceleration since the force that the cone is exerting on the air is changing due to excursion and different frequencies.
Now that the pressure in the box is too low, the speaker needs to **** in more air to be at equilibrium again. The process can repeat in reverse again and again until you get to equilibrium. That's why when you put a plastic bag in the port of your sub box to show off, it doesn't get sucked into the port, rather, it just flaps around. Air is being forced out and forced in succession.
This is the same reason why you cannot use too large of a port for a given volume because the speaker will not pressurize the chamber and will instead just shoot the air out the port acting like a horribly designed transmission line.
This is also why different speakers need different sized enclosures or not to be ported at all: if they cannot return that box to equilibrium (speakers with a high Qts value) quickly, the bass will be sloppy and the speaker can get out of control. A speaker that can create more pressure in the box is likely going to need a larger enclosure (larger diameter cone, or higher excursion) versus one that is smaller or moves less.
How does this pertain to slot and aero ports? An aero port allows the air to flow out of the enclosure more freely than does the slot port due to causing less turbulence, thus allowing for freer air flow for a given port area. That lets you get away with a smaller diameter port as he said, but the air is not being compressed better, rather, it's flowing more efficiently which is exactly the opposite. Unflared ports have little to no advantage.
Hopefully that helped you understand a bit about how ported boxes work and why you can't just throw any sub into a ported box and expect it to be loud.
dave
pv audio from a dif thread.
