Bandpass: the driver is sealed inside

In every other enclosure the cone faces the room. In a bandpass box it does not. The driver is mounted between two chambers and sealed away, and the only thing the room hears is the port.

That single fact explains everything else about the design, including the parts people find surprising.

Fourth order and sixth order

Fourth order: the rear chamber is sealed, the front chamber is vented. One port radiates.

Sixth order: both chambers are vented. Two ports radiate, and the passband gets narrower and louder still.

What you are buying

A bandpass box is the clearest illustration of Hofmann's Iron Law you can build. Efficiency, bandwidth and box size are traded against one another, and here the trade is made visibly and on purpose rather than as a side effect.

Narrow the passband and the level inside it rises. That is the whole proposition. A bandpass subwoofer can be several decibels louder than a vented box of the same size over the octave it covers, and correspondingly useless outside it.

The S parameter in the design formulas is the dial. A higher S gives a narrower, louder, peakier band; a lower one gives a wider, flatter, quieter one.

What you are paying

No output outside the band, on either side. The response falls away below the passband as well as above it, at 12 dB per octave each way for a fourth-order box. A sealed or vented box keeps producing something below its corner; a bandpass box does not, because the cone is sealed from the room and cannot.

The cone is hidden, and so are its problems. You cannot see or hear the driver directly. Cone excursion still matters, and a bandpass box unloads the cone outside its passband exactly where a vented box does, so watch the excursion plot rather than trusting the smooth response curve.

Port velocity is doing all the work. Every decibel leaving the box leaves through a port, so bandpass boxes chuff readily. Size the ports generously and watch the port velocity curve.

Out-of-band rubbish gets through anyway. The acoustic filter is gentle by electronic standards. Harmonic distortion produced inside the passband radiates out of the port along with everything else, and a poorly damped box will have audible upper-frequency leakage through the port walls.

Where it makes sense

Subwoofers, and particularly car subwoofers, where you want maximum level over one octave and have no interest in anything above it. Also public-address bass cabinets for the same reason.

It makes much less sense for a domestic full-range speaker, where the bass has to hand over smoothly to a midrange and the narrow band becomes a liability rather than a feature.