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Band pass subwoofer enclosures are a different animal. Where sealed and ported subs have a speaker directly firing into the area of a room, a band pass design is like two boxes in one. A speaker is mounted in an airtight box, which may or may not have a port. This speaker then fires directly into another airtight box which has one or more tuned ports firing into outside world.
Many computer speakers use a subwoofer of the band pass design. The advantage of a band pass design is improved efficiency over a sealed system no need for a crossover. The front chamber of the enclosure acts as a crossover, effectively filtering out the higher frequencies (except for a midrange port resonance that is oven higher than the main output). Their downfalls are complexity and the tendency to have 'muddier' bass than their sealed or ported brethren.