Is a bandpass box louder than a ported box?

Within the passband, a well-designed 4th order bandpass is typically 3 to 6 dB louder than an equivalent ported design. Outside the band, output drops off steeply.

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

A 4th order bandpass has a single ported front chamber. A 6th order has two ports in the front chamber. 6th order produces higher peak output and steeper rolloff but is significantly harder to design. 4th order is the standard car audio choice.

What drivers work best in a bandpass box?

Drivers with Qts 0.3 to 0.55 and Fs in the 20 to 50 Hz range. Avoid very high or very low Qts drivers, and avoid drivers with limited Xmax since bandpass designs allow higher excursion at the lower port tuning frequency.

Tools

Bandpass Subwoofer Box Calculator

A 4th order bandpass enclosure physically limits audio to a specific frequency band — the driver fires into a sealed rear chamber, which vents through a ported front chamber. Done right, it produces substantial SPL gain in the bass band. Done wrong, it sounds like everything was recorded inside a cardboard box.

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How it works

Enter driver specs
Type in your Thiele-Small parameters — Fs, Qts, and Vas — or pick a driver from the database. RokketBox uses the real parameters, no approximations.
Simulate frequency response
The circuit-domain engine computes rear chamber Qtc, front chamber tuning, and the combined bandpass response across 500 frequency points. See exactly where the band sits before cutting.
Export cut sheet
Every panel dimension, port length, and routing detail lands on a printable build sheet. Take it straight to the table saw.

4th order bandpass: how it works

In a 4th order bandpass, the driver sits inside a sealed rear chamber. The front face vents into a ported front chamber. The rear chamber acts as a low-pass filter rolling off high frequencies; the front port acts as a high-pass filter rolling off low frequencies. The combined result is a bandpass with significant output only between the two rolloff points. Passband location and width are determined by rear chamber volume, front chamber volume, and front port tuning frequency.

Chamber volume ratio and passband

The rear chamber volume primarily controls the upper cutoff frequency. Smaller rear chambers push the upper cutoff higher. The front chamber volume and port tuning determine the lower cutoff. A common starting ratio is 1:1.5 to 1:2 (rear to front) for drivers with Qts 0.3 to 0.5, but the optimal ratio depends on the specific driver. RokketBox uses Fs, Qts, and Vas to compute the optimal ratio and simulate the response.

Driver selection for bandpass

Drivers with Qts between 0.3 and 0.55 are the sweet spot for 4th order bandpass. Very low Qts drivers (below 0.25) produce narrow passbands with poor sensitivity. High Qts drivers (above 0.65) struggle because the sealed rear chamber Qtc becomes too high to produce a well-behaved rolloff. RokketBox simulates the actual response for your driver before you commit to cutting.

When bandpass makes sense

Bandpass enclosures are the right choice for dedicated SPL systems targeting a specific bass band, and for truck beds and trunks where cabin gain extends perceived bass. They are a poor choice for systems that need to reproduce above 100 Hz, for drivers outside the Qts 0.3 to 0.55 range, or for any build where the enclosure sees wide-range musical content.

Frequently asked questions

Is a bandpass box louder than a ported box?: Within the passband, a well-designed 4th order bandpass is typically 3 to 6 dB louder than an equivalent ported design. Outside the band, output drops off steeply.
What is the difference between 4th order and 6th order bandpass?: A 4th order bandpass has a single ported front chamber. A 6th order has two ports in the front chamber. 6th order produces higher peak output and steeper rolloff but is significantly harder to design. 4th order is the standard car audio choice.
What drivers work best in a bandpass box?: Drivers with Qts 0.3 to 0.55 and Fs in the 20 to 50 Hz range. Avoid very high or very low Qts drivers, and avoid drivers with limited Xmax since bandpass designs allow higher excursion at the lower port tuning frequency.

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