In a bandpass enclosure, the woofer resides between two separate chambers. Instead of playing directly into the listening area, the entire output of the speaker system is formed through vent(s). Vents act as an acoustic filter and limit the frequency response of the speaker to a specific bandwidth (passband). Adjusting the size of the enclosure and/or the dimensions of the vents, the frequency passband can be greatly altered. But why would anyone want to use a bandpass enclosure over a sealed or vented one? Well, besides the natural filter acting as a crossover, a bandpass enclosure can deliver considerable performance benefits in terms of efficiency and low-frequency extension that would likely not be possible in conventional designs of similar size.
This of course is a trade-off with regards to the parameters of the enclosure. When enclosure parameters are attuned for a narrow bandwidth, gains of up to 8 to 10dB in efficiency aren't uncommon within the passband of the subwoofer system. Adjusting the box parameters for a wider passband, low-frequency extension can be improved greatly, but at the expense of efficiency and 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 fourth or fifth order bandpasses. In a single reflex bandpass, the rear of the speaker resides in a sealed chamber while the front fires into a chamber tuned by a vent. The front chamber acts as a lowpass 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 reasonable transients.
Dual Reflex BandpassDual reflex enclosures are very similar in design to single reflex. The difference comes with a placement of a vent or vent in the rear chamber of the enclosure. This addition makes use of both the front and back wave produced by the speaker to improve the overall efficiency over that of a single reflex. The additional vent 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's a downside. This is apparent in the low-frequency roll-off that can be sharp, often in the range of 18 to 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 of 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 be degraded considerably.
Since all of the sound is poured through the vent, bandpass systems are vulnerable to the same issues as a vented enclosure. Nonlinearities in the vent mean higher distortion levels and a high group delay, making them less than ideal for sound quality.
In comparison to more conservative box designs, bandpass enclosures 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 vent proportions can create drastic changes in the response of the system. It's highly recommended that the design and construction of these boxes should be left to shops or people with extensive enclosure experience.