Stepping into the SPL lanes will cause most physicists to become confused. Immediately, they'll ask, "Where are all of the sealed enclosures?" You see, the majority of warped and extreme competitors taking top honors in SPL competitions today use ported enclosures, not sealed. Why? I think it is safe to say that many of these competitors don't actually know themselves, they just learn from trial and error. Cut and try.

So where does the answer to high SPL lie? The physicist is actually correct in theory to recommend a sealed enclosure. However, there is one small problem: subwoofers. Current subwoofer designs cannot move enough air due to excursion limitations. On the other hand, ported enclosures are slightly less dependent on excursion.

Ported systems are all around good performers, and most commercial home speakers use some type of ported enclosure. Automotive subwoofer manufacturers also like the ported enclosure, and most design drivers for this type of installation. The tuned port in these systems increases efficiency by nearly 3 dB in an optimum enclosure, and the roll-off frequency can be much lower, often by as much as 1/3 - 1 octave below a sealed enclosure. Think of the extra 3 dB as equal to the output you would get using twice the amplifier power on the driver. Add several ported drivers together and you can achieve impressive SPL's indeed! Because of the ports damping characteristics on the driver above Fb, distortion levels are also lowered because driver excursion is less. So, with nothing more than a properly designed optimum vented enclosure, you have very efficient bass reproduction with several advantages over an optimum sealed enclosure.

Different types of boxes will produce different types of bass:

Sealed boxes: For deep, precise bass
A sealed box is an airtight enclosure housing your subwoofer. A sealed box is best for any music that demands tight, accurate bass. Expect flat response (not excessively boomy), deep bass extension, and excellent power handling. Since a sealed enclosure tends to require more power than a ported box, use an amplifier with ample wattage for optimum performance.

Ported boxes: For forceful bass
Ported boxes use a vent (called a port) to reinforce low bass response. You get more output than you would from a sealed box at any given amplifier wattage. Some people prefer the sound of ported boxes for rock, heavy metal, or any hard-driving music. Ported boxes can deliver deeper bass than sealed boxes, though they need to be much larger than sealed enclosures to accomplish that.

a ported box is usually about +3dB more efficient at the frequency of the tuned port(s). These boxes require less power than sealed to reach the same SPL, but below their tuned frequency, they are very hard to control, and require great amounts of power to produce those below-port frequencies. Ported enclosures also add coloration to the sound, which detracts from their accuracy.

Well, you're not missing anything really... what you're noticing is the advantage of a vented allignment over a sealed allignment. Sealed boxes have only 1 source of spl, being the driver. Vented speakers have two sources, 1 being the port, the other being the driver.

In some smaller rooms, it makes more sense to go with a sealed subwoofer because they generally(w/o highpass) exhibit a 2nd-order roll-off after Fb (tuning frequency or enclosure resonance). Vented systems have a 4th order roll-off after Fb and an increasing danger of exceeding xmax after the tuning frequency. Smaller rooms will help the sealed subwoofer in the lower frequencies with its own contribution to spl called room gain. Also, since the roll-off is more gradual with the sealed subwoofer it may mesh better with the room gain. Vented boxes roll-off twice as fast below the tuning frequency.

in SPL comps ported is virtually te only kind of box used, tuned to the resonant frequency of the car you gain substantially over sealed boxes.

here's some facts for those wanting to design a sound system for more bass, especially deciding between sealed vs. ported boxes and amp wattage.

let's take a sub, a JL Audio 10" (10W0), for example. it handles 125W RMS and can be used in a sealed or ported box. it will work great in a 0.75 cuft sealed box and a 1.25 cuft ported box. for any given wattage, the sealed box will hit tighter and lower, while the ported will be louder. u can modify the sound of either box by changing box size and/or port size.

let's say the sub puts out 100dB at 125W in a sealed box (its probably lower than that). if wattage stays the same, the ported box will give a 3dB increase, so u'll get 103dB with the ported version.

for a sealed box, to increase SPL by 3dB, u have to double the power. so u can get 103dB from sealed if u feed it 250W. but now the speaker cant handle that much power.

so u have to decide what type of bass u want, accurate vs. loud. then decide how u want to increase SPL, by porting or by doubling the power?

one last thing, all else being equal, doubling the amount of subs will increase SPL by 6dB.