Bandpass: loud in a narrow band, harder to get right

Sealed rear chamber, ported front. Enter the driver and the tuning you want; get both chamber volumes and the port to build. 4th-order only — 6th and 8th need real simulation software, and the note below says why.

This is a STARTING POINT, not a validated design. It does not model the true coupled bandpass frequency response — that requires simulation software (WinISD, BassBox Pro). Run whatever this produces through real simulation before cutting any wood. A sealed or ported box has one air volume and a closed-form answer. A bandpass has two chambers that load each other through the driver, so the response depends on both at once and on how they interact — which is why the usable passband is roughly 1 to 1.5 octaves wide and why getting it wrong costs you output rather than just shifting the tuning.

Driver & rear (sealed) chamber

Pick a driver from the database (optional) Vas (driver's rated equivalent volume, ft³) Qts Target Qtc for the rear chamber

0.707 is a starting value, the same default as the sealed-box formula elsewhere in this app. In a bandpass the passband shape also depends on the front chamber, so it is not a promise of a flat response. Higher Qtc = peakier and a smaller rear chamber.

Front (ported) : rear (sealed) volume ratio

Sourced guidance disagrees between roughly 1.2:1 and 3:1 — this is a real, documented disagreement, not a single right answer. Try a few values in this range; only real simulation will tell you which one actually lands on your target frequency for your specific driver.

Fs (driver's free-air resonance, Hz) — optional Sd (cone area, cm²) — optional

Optional. Fs adds the rear chamber's own resonance and where your tuning sits against it; Sd checks the port area against the cone. Picking a driver fills both when they are published.

Front (ported) chamber tuning

Target port tuning frequency (Hz)

Guidance found: tuning near 0.9x driver Fs narrows the passband (tighter bass); near 1.1x widens it; up toward 1.6x raises peak output at the cost of looseness/accuracy. Typical targets are 44-46 Hz (low end), 47-52 Hz (daily) and 55-65 Hz (SPL). This is a real tradeoff, not a default to blindly copy.

Port diameter (in) Number of ports

Port end condition

One flanged, one free (typical) Both ends flanged Both ends free (rare)

Result (starting point only)

Fill in the driver parameters and tuning target, then hit Estimate.

Next step

Both chamber volumes above are net air, so turn each into a build size with the Net Volume Helper: the driver's displacement goes in the rear chamber, the port tube's outside volume and any bracing in the chamber they sit in. The Port Calculator is for a slot port only; the round port length is already above. Check the figures against real simulation software before cutting — see the warning at the top of this page for why that matters more here than on a sealed or ported box.

Why 6th- and 8th-order are out of scope: 8th-order is genuinely SPL-competition-only territory, and 6th-order — especially series-tuned, with 7 interacting variables — is complex enough that a hand-calculated starting point wouldn't be defensible. Both need real simulation software, not a general calculator like this one.