Sounds below the 400hz range do not require a left, and right channel, simply because the human ear can't distinguish which side the sound is coming from. Therefore, designs have lead to both audo channels being led into one speaker (dual wound voice coils), or simply placing two speakers into a bass enclosure. This is also why all surround sound systems today only have one subwoofer for bass.
For many years now, bandpass enclosures have been quite popular both for their aesthetics and performance. A properly designed, constructed and implemented bandpass enclosure can and often will out-perform the same driver or drivers in a more conventional sealed or ported design in terms of sheer output and/or low-frequency extension. A bandpass alignment also allows the installer to funnel a potentially large amount of low-frequency energy into the vehicle's cabin through a relatively small opening. This can be particularly useful in some vehicles such as European sedans whose tank-like construction does not facilitate a satisfactory transfer of low-frequency energy from the trunk into the cabin.
Cubic foot for cubic foot a single reflex-bandpass, characterized by a sealed "rear" chamber(s) and ported "front" chamber(s), typically offer better transient response and low frequency extension than will its more elaborate cousins. In addition, while still a fairly difficult system to construct properly, single-reflex designs are a bit more forgiving of minor errors in calculation and assembly when compared to more sophisticated bandpass types. All things considered, we have found that single-reflex designs offer the best combination of reasonable enclosure size, good transient response and predictable behavior.
So how do they work?
A bandpass enclosure is, by definition, simply a sealed enclosure with an acoustical filter in front of it that serves to limit the upper-end of the driver's frequency response. This natural limiting of the high-frequency response of the system makes the selection of mid-bass drivers critical. If your vehicle cannot fit larger midbass drivers (such as a 6 1/2" or larger) then a bandpass enclosure is probably not the best choice for you. Using a bandpass enclosure with insufficient mid-bass reinforcement will lead to sluggish, sloppy, muddy, impact-less low frequency response. In short--it will sound like a soggy pancake hitting a cardboard box.
Once adequate mid-bass reinforcement has been selected to complement the sub-system it will be necessary to add additional electronic filtering to further limit the upper frequency output of the enclosure. Contrary to popular belief, a bandpass enclosure (of any type--single reflex, dual reflex, series-tuned, etc.) does require the use of an electronic crossover to achieve optimum performance since the acoustical low-pass filter is not a very effective filter. What proponents of "crossover-less" bandpass enclosures neglect is that there is a considerable amount of high frequency output (called "out-of-band noise") that can get to be quite annoying. It is for this reason that all bandpass enclsoures be supplemented with an electronic crossover, or, and amplifier that has biult in frequency cut-offs at the mid bass range (to 450hz).
Please go to:http://www.jlaudio.com/tutorials/bandpass/
This will explain the above, plus symetrics and terminalogy, and more
on what they do, and how they work.