Bandpass subwoofer box explained: 4th order vs 6th order

A bandpass subwoofer box mounts the driver entirely inside the enclosure, between two chambers, and lets sound out only through a port. It can deliver more output than a sealed or ported box over a limited range, in exchange for narrower bandwidth and a design that must be simulated carefully.

How a bandpass box works

In a sealed or ported box, the front of the cone radiates straight into the room. In a bandpass box, the cone fires into a chamber, and that chamber's port is the only thing you hear. The driver never sees the outside world directly.

The enclosure acts as an acoustic band-pass filter. The chamber behind the driver controls the low end, much as a sealed or ported box would. The ported chamber in front acts as a low-pass filter: the air mass in the port and the springiness of the chamber air roll off output above a certain frequency. What comes out is a band of bass with slopes on both sides.

That built-in roll-off is the first practical benefit. Driver noise, cone breakup and midrange leakage stay inside the box, and the upper slope helps a car or home crossover do its job.

4th order bandpass

The 4th order design is the simpler and more common one. The driver sits on a wall dividing the box into two chambers:

• Rear chamber: sealed. It sets the driver's spring loading, like a sealed box.
• Front chamber: ported. The port tuning sets where the passband sits, and the chamber volume sets how wide it is.

It is called 4th order because the combination behaves like a fourth-order filter in total: roughly a second-order (12 dB/octave) roll-off below the passband and a second-order roll-off above it.

Strengths: good power handling at low frequencies, since the sealed rear chamber controls excursion even below the passband; natural high-frequency filtering; and decent output in a modest box when the passband is kept reasonably narrow.

Weaknesses: the box is larger and more complex than a sealed box of similar extension; the front port can be long and is prone to noise if undersized; and the passband is not as wide as a good ported box unless you accept lower efficiency.

6th order bandpass

A 6th order box ports both chambers. Each chamber and port pair is tuned to its own frequency, usually one near the bottom of the passband and one near the top. (A variant adds a third port or a passive radiator, but the dual-ported version is the classic.)

Strengths: the highest efficiency potential of the common box types, and a wider passband than a 4th order design for the same gain. Competition builds aimed at a specific frequency range often use it.

Weaknesses: steeper slopes and more sensitivity to errors. Two tunings and two volumes interact, so a mistake in one shifts the whole curve, often producing a dip or a peak in the middle of the band. Below the lower tuning, the driver unloads just as in a ported box, so a subsonic filter is essential. Group delay is higher, and transient response tends to be the weakest of the common types.

The efficiency vs bandwidth trade-off

Bandpass boxes do not create energy. They redistribute it. A driver has a certain efficiency bandwidth product, and the box decides how that is spent:

• Narrow passband, high gain. Make the front chamber small and the passband narrow (about an octave wide) and output inside that band can be several dB above what the same driver manages in a sealed box. Outside it, output falls off quickly. Great for a burp contest at one frequency; poor for music that spans 25-80 Hz.
• Wide passband, low gain. Make the front chamber bigger and the passband widens to two octaves or more, but peak output drops toward, or even below, what a well-designed ported box gives.

This is why many builders end up with a ported box: it already gets a good chunk of the bandpass efficiency benefit near tuning, with a simpler build and a wider, flatter response. The sealed vs ported guide covers that comparison.

Why a bandpass box needs simulation

A sealed box has one variable that matters (volume), and a ported box has two (volume and tuning). A 4th order bandpass has three, and a 6th order has four. They interact. A 10% error in the front chamber of a 6th order box can tilt the passband or open a hole in the middle of it.

So design one this way:

1. Check the driver. Bandpass designs usually prefer drivers with low to moderate Qts and solid Xmax. The Thiele/Small parameters guide explains what to look for.
2. Simulate with a bandpass model, such as the 4th and 6th order modes in the BassDesign calculator, using accurate parameters, and decide the passband you actually want before chasing gain.
3. Size ports for velocity. The front port carries the entire output of the system, so it needs generous area. The port noise guide shows how to check air speed.
4. Account for everything inside. Driver displacement, port volume and bracing all come out of the chamber volumes. Work in net volume and add these back for gross dimensions.
5. Build accurately and seal well. A leak in the sealed rear chamber of a 4th order box changes its alignment.
6. Use a subsonic filter for any design with a ported rear chamber.

Before committing to a bandpass build, it is worth simulating the same driver in a sealed and a ported box as a baseline. If the bandpass only beats the ported box by a decibel or two over a narrower range, the simpler box is usually the better choice. BassDesign models sealed, ported and both bandpass types with the same driver, and you can pin each design and overlay them on one graph, so the comparison takes a couple of minutes.

When a bandpass box makes sense

• You want maximum output in a defined band, for example 35-60 Hz for loud bass in a vehicle, and you accept less extension.
• The sub must be hidden or isolated, since only a port opening needs to face the listener. It is a common choice for sub enclosures firing through a trunk pass-through.
• The driver needs protection, since the cone is enclosed and out of reach.

Skip it if you want deep, flat extension for music or home theatre with minimal effort, if space is tight, or if you cannot build to accurate dimensions. In a car, remember that cabin gain already lifts the low end (see the car cabin gain guide), which often makes a simple sealed or ported box the more balanced option.