Dual reflex enclosures are very similar in design to single reflex. The difference comes with the placement of port(s) in the rear chamber of the enclosure. This addition makes use of both the front and back wave of the speaker to improve efficiency over that of a single reflex. The additional port(s) also allows for extended low frequency response. In a dual reflex, the cone motion of the speaker is generally very well controlled within the bandwidth. But, as with everything there is a downside. This is apparent in the low frequency roll off that can be sharp, often in the range of 18-24dB per octave. This steep roll-off in the low frequency implies that a dual-reflex usually has to be larger in size to produce the same low-frequency extension as a single-reflex design. The low frequencies are a direct effect of the roll-off of the enclosure design; and as the enclosure is made larger to compensate, the transient response can degrade considerably.
In comparison to more conservative box designs, bandpass enclosures are can be very complex to design and construct. Their complexity leaves little or no room for error without suffering consequences. The slightest miscalculation in volume size or port length can create drastic changes in the response of the system. It is highly recommended that the design and construction of these boxes should be left to shops or people with extensive enclosure experience.
When it comes time to build an enclosure, it is important to keep in mind that there is no such thing as a "magic box." Every enclosure has its benefits that make it that better than the next design. No one box will yield the highest SPL, best transient response, smoothest frequency response, as well as power handling. So when it comes time to build the enclosure, be sure that you use the recommendations provided by the manufacturer to assure that the speaker performs the best that it possibly can.
In part two we'll explore alternative enclosures. Stay tuned.