Slot port vs Standard port

Fs is the point of resonance for the physical parts that make up the loudspeaker.  It is where the speaker's impedence is maximum (usually on the order of 30 ohms or higher.)  Fs is where the loudspeaker parts become balanced and are not controlled properly by the motor structure.  The cone "rings" or vibrates and its usable output drops to near zero, the excursion reaches max and distortion goes through the roof.  In general, you want to avoid driving a loudspeaker at Fs (there are exceptions, such as an ELF system which operates below Fs, but that is way beyond the scope of this question.) 

Fb is the resonant frequency of an enclosure.  In acoustic suspension (sealed) systems it is always higher than Fs for a properly designed enclosure/speaker system as the air inside the enclosure resists speaker motion and helps prevent the cone from ringing.  Actually the proper term for a sealed speaker/enclosure system resonance is Fc.  Usable output below Fc rolls-off at a predictable -12db/oct.

Using a ported enclosure (bass reflex) allows the designer to push F3 (the -3db response point) down closer to Fs.  The system will "boom" at the Fb tuning frequency, as sound at that point is being produced by the port not the loudspeaker.  The roll-off after F3 is about -24db/oct.  Ported systems are generally more efficient than sealed and can produce higher SPL, but the trade-off is that a selaed system protects the driver (loudspeaker) from ever operating at Fs and being potentially damaged by exceeding its excursion limits since the Fc point is higher than Fs.  In a ported system, the driver becomes unloaded below Fb and can very easily be driven to damage.  Ported subwoofers generally need to be protected from over-excursion by a high-pass filter set at or about Fb.

For subwoofers, a properly designed slot port will sound no different than a properly designed round port.  For midrange frequencies, a round port generally sounds more musical (the tone the port produces tends to sound fuller and less like air being pushed out of a port.) 

Hope this helps.

Thanks DYohn, yes that helps alot.

Now I know I don't need to spend the extra money to have a slot port box made. I can save about $100 by going with an old fashioned bass reflex box (I'm having it built).

Thanks again!