| Growler is on the right track, but as he suspected a more detailed explanation is on its way.
The port + the volume of air in the box forms a "Helmholtz resonator". Now that sounds pretty technical, but I'm betting it's something you've encountered before even if you didn't know the name for it. Ever blow across the top of a beer bottle? Wuuuuuuuh. That's a Helmholtz resonator, formed by the neck of the bottle and the volume of air in the bottle. It resonates primarily at a certain frequency, but also puts out some energy on either side of that resonant frequency. Reduce the amount of beer in the bottle (thus increasing the volume of air), and the pitch goes down. If you could increase the length of the neck while keeping its diameter the same, that would also lower the pitch.
Back to our bass cab. Instead of blowing across the port from the outside of the box, the woofer in effect blows across the port from the inside of the box. Wuuuuuuuh. We designer types "tune" the port dimensions so that the Helmholtz resonance is a bit lower in frequency than where the woofer would normally roll off in that size cab, and thus we get deeper bass than we otherwise would have. We have to keep the port area wide enough for the airflow, and we have to pay attention to fartout, but if we do a decent job of juggling all the tradeoffs then the result is increased usable bass output compared to a sealed cab with the same woofer.
Last edited by DukeLeJeune : 02-20-2011 at 07:02 AM.
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Question about cabs 
