Bandpass Subwoofer Box Design: 4th Order vs 6th Order Explained

A bandpass subwoofer box trades bandwidth for output. It hides the cone between two chambers and lets only a band of frequencies escape through a port, which is why a well-built one can hammer in its tuned range and disappear everywhere else. The two designs you will actually see are 4th order and 6th order, and the difference between them is exactly how many chambers are ported and how much you are willing to give up in bandwidth and forgiveness to gain output.

This is the bandpass reference for the Audio Intensity enclosure library. If you are still deciding between enclosure types in the first place, start with our subwoofer box guide and the sealed vs ported breakdown. For the port math that bandpass designs depend on, keep the port area calculator and the box volume calculator open while you read.

What is a bandpass subwoofer box?

A bandpass box is an enclosure where the driver is mounted on an internal baffle between two sealed-or-ported chambers, and the cone does not face the cabin at all. Every bit of bass you hear comes out of a port, not off the cone. Because the chambers and ports only pass a limited range of frequencies, the box behaves like an acoustic bandpass filter: it rolls off both below and above a center band, and it boosts output inside that band.

That is the whole trade. A sealed box plays wide and flat and accurate. A ported box extends the low end and adds efficiency near tuning. A bandpass box takes that idea further and concentrates the driver's energy into a narrower window, buying in-band loudness with the low and high extension it throws away.

How is a 4th order bandpass different from a 6th order?

The difference is how many chambers are ported. A 4th order box has a sealed rear chamber and a ported front chamber, so a single port carries the output. A 6th order box ports both the rear and front chambers, so it has two tunings working together. That one change drives everything else: bandwidth, peak output, group delay, and how badly a small build error hurts you.

4th order: sealed rear, ported front

The 4th order is the bandpass design most people should actually run. The sealed rear chamber controls the cone, the ported front chamber sets the output band, and the system rolls off at roughly 24 dB per octave on each side. It gives you more output than a sealed box of similar size while staying relatively forgiving to design and build. You still give up extension and you still get more group delay than sealed, but it is the version that survives real-world tolerances.

6th order: ported rear, ported front

The 6th order ports both chambers and rolls off at roughly 36 dB per octave. Done right, it is louder than a 4th order across a narrower passband and more efficient in that band. Done slightly wrong, it builds a sharp peak, rings, and sounds one-note. It demands modeling, accurate volumes, and accurate port tuning, and it punishes guesswork. This is the design you see in SPL and burp builds, not in a daily SQ car.

Which one is louder, and which sounds better?

A 6th order is louder in its passband, and a sealed box sounds better everywhere. Those are two different questions, and bandpass only wins the first one. If you measure peak SPL in the tuned band, the 6th order usually takes it, the 4th order is close behind, and both beat a comparable sealed box in that narrow window. If you judge accuracy, transient attack, and how the bass tracks the music, bandpass loses to sealed every time because the cone is filtered and the output carries more group delay.

This is also why we steer SQ customers away from bandpass. If your goal is staging, detail, and bass that disappears into the rest of the system, a sealed box and a properly tuned DSP get you there. Bandpass is a tool for output in a target band, not for fidelity.

When should you run a bandpass box, and when should you skip it?

Run a bandpass box when you want maximum output in a specific band and you are willing to give up extension and accuracy to get it. Skip it when you care about sound quality or want one box that plays everything well.

Good fits for bandpass:

• A daily driver who wants more slam than sealed without the size of a big ported build (4th order).
• SPL and burp builds chasing a number in a known band (6th order).
• A high-EBP, low-Qts driver that already models well in vented alignments.

Skip bandpass when:

• You are building for sound quality. Choose sealed.
• You want deep, clean extension across a wide range. Choose ported.
• You cannot model the driver or build to spec. A guessed bandpass box is worse than a simple sealed box.

How is a bandpass box designed and tuned?

You model it to the driver, then you build it to the model with no drift. A bandpass design is defined by the rear chamber volume, the front chamber volume, and the port tuning of each ported chamber. Each ported chamber is a Helmholtz resonator, and the interaction of those tunings sets where the passband sits, how wide it is, and how flat it stays. You start from the driver's published Thiele-Small parameters, not from a generic plan, because the same box volume that is perfect for one 12 is wrong for another.

A quick screen before you commit: a driver's EBP, equal to Fs divided by Qes, tells you whether it leans sealed or vented. An EBP above roughly 100 points to ported and bandpass alignments, while a value below 50 wants a sealed box. Run your driver through our EBP calculator and review the Q factor guide before you model anything.

This is where the build matters as much as the math. We cut bandpass enclosures on our ShopSabre CNCs to a 0.005 inch tolerance, out of 3/4 inch Langboard Elite MDF (48.5 lbs per cubic foot, 200 psi internal bond, 410,000 psi MOE). The reason is simple: a bandpass box only does what it modeled if the chamber volumes and port lengths are exactly what you drew. A hand-cut box with loose seams and approximate volumes will not land on the modeled response, and on a 6th order that gap is the difference between a flat, loud passband and a ringing peak. If you want one built to your driver, contact us with the make and model and we will model it first.

Why do most bandpass boxes underperform?

Most disappointing bandpass boxes failed for one of two reasons: they were never modeled to the driver, or the build drifted off the model. People copy a plan built for a different sub, or they eyeball the volumes, or they cut a 6th order with a port that is close but not right. Because bandpass response depends on exact volumes and tuning, close is not good enough. A tenth of a cubic foot or an inch of port length shifts the passband and stacks up peaks and dips you then try to EQ out, which wastes power and headroom.

If a bandpass box sounds peaky, boomy on one note, or weaker than expected, the answer is almost never a different amplifier. It is the enclosure. Re-model to the actual driver, verify the real internal volumes, and confirm the port tuning before you blame anything electrical.

Frequently Asked Questions

Where to go next

If you are still choosing an enclosure type, read the subwoofer box guide and the sealed vs ported comparison first, since most builds belong in one of those two rather than a bandpass. If you have decided on ported, the ported box guide and the port area calculator take you the rest of the way.

When you are ready to have a bandpass enclosure modeled and cut to your exact driver, contact us with the subwoofer make and model. We model it to the published parameters before we cut anything.