ConclusionWhen it comes time to build an enclosure, it's important to keep in mind that there's no such thing as a "magic box." Every enclosure has its benefits that make it better than the next design. No one box will yield the highest SPL, best transient response, lowest group delay, best frequency response and the best power handling. When it comes time to choosing your enclosure, be sure that you use the recommendations provided by the manufacturer to assure that the speaker performs the best that it possibly can.
Magical Mystery BoxIt stands to reason that if a truly perfect loudspeaker system exists that sounds absolutely identical to the original acoustic performance, there also must be a perfect subwoofer to go with that system. That subwoofer would then be mounted in a perfect enclosure. So what type of subwoofer enclosure would you find in a "perfect" loudspeaker system?
OK, first off it's highly unlikely that there will ever be a universally recognized perfect loudspeaker system any more than there's a perfect martini: It's all shades of gray and subject to the variations of human perception and opinion. Still, we'd all like to believe that one type of box format is superior to another - vented better than sealed, sealed better than passive radiator or whatever. I certainly have my opinion about that, and I'm sure all of you have yours, but from an engineering standpoint, it's really about tradeoffs. If we leave opinion behind, each enclosure type, sealed, vented, passive radiator or bandpass has its good points and its downside.
VentedLet's start with vented boxes. Ported enclosures are easy to design, have very good low-frequency performance (20Hz isn't a problem for a vented box) and are theoretically more efficient, so it would be tempting to pick this type of box as the best enclosure. But, as with all things in life, there's no free lunch. Generally, when ports are large enough in diameter to move air efficiently in and out of the box, they're too long to fit in the box, which is another way of saying that virtually all-usable port sizes are very non-linear especially at high SPLs. Ported enclosures also suffer from higher group delay than sealed boxes. Group delay is engineering speak for how accurate a woofer reproduces the incoming signal it receives and is usually described in terms of transient performance. Compared to sealed boxes, vented boxes are generally more non-linear and less accurate.
SealedThis must mean that sealed boxes are better? Again, no free lunch. Sealed boxes are likewise easy to design and also very easy to get good transient performance. Coming up with a 12" woofer with a low box Qtc of 0.7 is a piece of cake, but getting that same box to go down to 20Hz - an easy task for a vented box - is a serious struggle for a sealed box. You generally need a very low-resonance woofer, which creates mechanical design issues or you need some kind of amplifier manipulation in the form of low-frequency boost or servo control.