OK, but then why do most bass speaker enclosures have a reflex port?

This is a very good question, and I anticipated it.

A bass reflex (ported) enclosure is one of the two possible ways to build an enclosure. A sealed-box enclosure is also called an "Acoustic Suspension" enclosure.

Some speakers are designed to work in ported enclosures, others are designed to work in sealed boxes (but you have to get the cubic volume of the sealed box correct).

With a ported enclosure, the "port" is specifically tuned to allow only a certain volume of air to escape per second. There is still a little bit of backpressure on the speaker. In a ported enclosure, the bass radiates from both the speaker and the port. The port usually emits frequencies in a certain narrow range, depending on how it's tuned. Keep in mind, though, that a ported enclosure is still an enclosure- the speaker's not just sitting out in the air.

A ported enclosure will sound louder than a sealed enclosure, requiring less power to drive it. But it will sound less accurate, more "boomy", its frequency response will not be flat across the speaker's range (it'll have a spike around the port frequency), and it won't extend to very low sub-bass notes. Ported enclosures are popular because they can get louder with less amplification.

A sealed enclosure (when properly sized to the speaker), will require a bit more power to drive properly. But it will have a flat frequency response, it will extend to lower bass notes, and it will sound tighter and more accurate.

I have firsthand experience with this- I had a ported, pre-assembled, self-powered subwoofer in my hatchback for a while. After it got stolen, I assembled my own sealed-box unit from component parts. For about the same amount of money as I'd paid for the ported unit, I got a sealed-box unit that sounds orders of magnitude better, and that's what I'm using now. I was surprised at how much better the sealed-box unit sounds.

Okay, I lied above. There's actually a third way to create a bass enclosure: it's called a "Bandpass" enclosure, where the speaker sits between two chambers inside the box, and bass radiates only from the port(s). This design has the problem of narrowing the frequency response to a very narrow range- the range of the port(s). It's fine for people who just want a loud "boom" and aren't interested in the nuances of all the low frequencies.

Tony Fabris
Empeg #144