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Bandpass Box Calculator (4th Order) with Response Curve

Sealed rear and ported front chamber from your sub’s Fs, Qts and Vas: volumes, tuning, port length and the curve.
Curve
On
its
own
In
a
car
On its own
In a car
Rear chamber (ft³)
Front chamber (ft³)
Tuning (Hz)
Subs in this box: 1
Fs, free-air resonance (Hz)
From your sub’s spec sheet.
Qts
Vas (ft³)
1.5 ft3 or 42 L. A bare number is ft³.
Rear chamber Q (Qbp)
0.6
0.707
0.8
0.9
1
0.707 is flat at the sub’s own level. Higher is louder over a narrower band.
Port diameter (round)
Net volume (both chambers)
1.19 ft³ (33.6 L)
Rear chamber (sealed, net)
0.71 ft³
Front chamber (ported, net)
0.48 ft³
Front chamber tuning (Fb)
53 Hz
Plays from (−3 dB)
32.8 Hz – 85.8 Hz
Level vs the sub alone
−0 dB
Port length, one 3" (76 mm) round port
11 15/16" (303 mm)
Formula
Vr = Vas ÷ ((Qbp ÷ Qts)² − 1), Vf = 2 × Qts² × Vas, Fb = Fs × Qbp ÷ Qts
With
Fs = 30 Hz, Qts = 0.4, Vas = 1.5 ft³, Qbp = 0.707
Starting points, not guarantees. Your sub’s manual wins: check its recommended box and tuning.

How it works

Type Fs, Qts and Vas, pick the rear chamber Q, then drag either chamber or the tuning and watch the curve move.
Example: Fs 30 Hz, Qts 0.40, Vas 1.5 ft³ at Qbp 0.707: rear 1.5 ÷ ((0.707 ÷ 0.40)² − 1) = 0.71 ft³, front 2 × 0.40² × 1.5 = 0.48 ft³, tuned to 30 × 0.707 ÷ 0.40 = 53.0 Hz.
That box plays from 32.8 to 85.8 Hz at the sub’s own level. One 3 in round port in the front chamber is about 11.9 in long.

Frequently asked questions

What is a 4th-order bandpass box?

The sub sits on a wall between two chambers: a sealed one behind it and a ported one in front. Only the port plays, so it is loud over a band and falls off on both sides of it.

Is a bandpass box louder than a ported box?

Over its band it can be: at Qbp 1 it plays 6 dB above the sub on its own, over a narrower band. At 0.707 it is flat at the sub’s level, 32.8 to 85.8 Hz for the example sub. Outside the band it falls 12 dB per octave.

Which chamber gets the port?

The front chamber, the one the cone faces. The rear chamber is sealed. "Size the port" works out other port sizes; "Build" turns each chamber into panels and a cut list.