When enclosure parameters are attuned for a narrower bandwidth, gains of up to 8-10dB in efficiency are not uncommon within the bandwidth of the subwoofer system. Adjusting the box parameters for wider bandwidths, low frequency extension can greatly be improved, but at the expense of efficiency and good transient response. Using an intermediate bandwidth can create a compromise between these characteristics.
Bandpass enclosures come in two flavors: single and dual reflex. The more popular of these is the single-reflex bandpass, otherwise referred to as a sealed rear chamber bandpass. In a single-reflex bandpass, the rear of the speaker resides in a sealed chamber while the front fires into a chamber that is tuned by a port. This front chamber acts as a low-pass filter, which acoustically limits the high frequency response of the speaker. The roll-off of this type of bandpass is typically 12dB per octave, much like that of the common sealed enclosure with better overall transient response.
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 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 there is no such thing as a "magic box." Every enclosure has its benefits. No one box will yield the highest SPL, best transient response and smoothest frequency response, as well as power handling. So when it comes time to build the enclosure, be sure you use the recommendations provided by the manufacturer to ensure the speaker performs the best it possibly can.
Dual Reflex BandpassGood: Can be even more efficient than a single reflex bandpass; excellent damping at resonance; broad bandwidth with extensions at low frequencies.
Bad: Extremely difficult to design and construct; not very forgiving of construction errors; like a ported enclosure it looses woofer control below f3; steep low frequency roll-off (18 to 24dB); very large enclosures; stressful on speaker joints and suspension.
Single Reflex BandpassGood: Can be very efficient within the bandwidth; controlled cone excursion at low frequencies; increased power handling within the band.
Bad: Distortion hard to hear so it's very easy to damage woofers; relatively large enclosures; difficult to design and build; sensitive to design and construction errors; stressful on speaker joints and suspension.