Bandpass box calculator

A bandpass box hides the driver inside and lets only the ports speak. Done well it is the loudest thing you can build over a narrow band; done badly it is a boomy box that eats drivers quietly.

The two kinds

4th-order (single-reflex) bandpass — a sealed rear chamber and a vented front chamber. The cone fires into the front chamber; only the front port radiates into the room. The rear chamber sets the low corner, the front chamber and its vent set the high corner.

6th-order (double-reflex) bandpass — both chambers are vented, and both ports radiate. Steeper skirts on both sides and more output in the passband, at the cost of a much fussier design: the two tunings interact, and getting them wrong produces a double-humped response that sounds worse than a plain ported box.

The trade you cannot escape

Bandpass alignments obey a conservation rule: gain times bandwidth is roughly fixed for a given driver. You choose where on that curve to sit.

A narrow bandpass really is louder — the gain is real, not a measurement trick — but it is louder over an octave and a half, and it has nothing above or below. Music that lives outside that window simply is not reproduced.

How the chambers divide the work

• The rear (sealed) chamber sets the low end. Larger rear chamber, lower low corner, less gain. It behaves exactly like a sealed box — Vb from a target Qtc, same formula, same meaning.
• The front (vented) chamber sets the high end. Its volume and vent tuning set the upper corner and how sharply it falls.
• The ratio between them sets the gain. A small rear chamber against a large front chamber is a narrow, high-gain box. Even chambers are wide and flat.

What goes wrong

1. Hidden excursion. A bandpass cone can be at the end of its travel inside a chamber you cannot see, on frequencies you cannot hear, while the box sounds fine. Check excursion against Xmax across the whole band — not just in the passband — before building.
2. Vent air speed, twice. A bandpass box does all its radiating through vents, so they carry every bit of the output. Air speed matters more here than in a plain ported box, and a 6th-order has two vents to check.
3. Chamber leaks between the two chambers. The divider panel has to be as airtight as the outside of the box. A leak there turns a bandpass into something with no name and no model.
4. No subsonic filter. Same as a ported box, and for the same reason, but worse because you cannot hear the driver complaining.
5. Treating the passband gain as free. It is borrowed from the bandwidth. If you need 30–80 Hz flat, a bandpass is not the shape to be reaching for.

When a bandpass is the right answer

It is right when the job really is a band: a car subwoofer crossed over at 80 Hz and never asked for anything above it, a sub in an install where the driver has to be hidden and protected, or an SPL build where the metric is one number at one frequency. It is the wrong answer for a full-range cabinet, for anything that needs a clean handoff to a midbass, and for a first build — the second chamber doubles the number of things that can be built slightly wrong.

Frequently asked questions

Is a bandpass box louder than a ported box?

Inside its passband, yes — typically 3–8 dB more for the same driver and power. Outside it, no: there is nothing above the upper corner and it falls away steeply below the lower one.

The gain is borrowed from bandwidth rather than created. Across a wide range, a ported box gives more usable bass.

What is the difference between 4th-order and 6th-order bandpass?

4th-order has a sealed rear chamber and a vented front chamber, so one port radiates. 6th-order vents both chambers, and both ports radiate.

The 6th-order gives steeper skirts and more passband output, but the two tunings interact — an error in either chamber gives a double-humped response. The 4th-order is far more forgiving and is the one to build first.

Do bandpass boxes damage subwoofers?

Not inherently — but they hide the warning signs. With the driver sealed inside and only the ports radiating, you cannot hear a cone bottoming or a suspension at its limit the way you can in a ported box.

Excursion can be past Xmax outside the passband while the box sounds perfectly normal. Model excursion across the whole range, and fit a subsonic filter below the lower tuning.

Why does my bandpass box sound one-note?

Because the passband is too narrow. A high-gain alignment concentrates output into a small range, so everything comes out emphasised at roughly the same pitch.

Widen it: a larger rear chamber lowers the bottom corner, and the front chamber and its tuning move the top. You will lose a few dB of peak output — that is the trade working as designed.

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