Speaker box calculator
Sealed and ported enclosure design from three Thiele/Small parameters — box volume, tuning, port length, and an honest response preview.
Volumes are net internal volume — add driver displacement, bracing and port volume on top. Fill/stuffing makes a sealed box act ~10–15% bigger.
Sealed boxes: pick a Qtc
A sealed box turns the driver into a second-order high-pass with total Q of Qtc. The box volume follows directly: Vb = Vas / ((Qtc/Qts)² − 1), with Fc = Fs·Qtc/Qts. Qtc 0.707 (Butterworth) is maximally flat; 0.577 trades extension for the best transient behavior; approaching 1.0 sounds punchier but rings and rolls off higher. Qtc below your driver's Qts is physically impossible — the tool flags it.
Ported boxes: volume and tuning
A ported (bass-reflex) box adds a Helmholtz resonator that extends response roughly half an octave deeper for the same driver, at the cost of steeper rolloff (24 dB/oct) and less control below tuning. The classic QB3-family starting point for drivers with Qts ≈ 0.2–0.4: Vb ≈ 15·Vas·Qts^2.87, Fb ≈ 0.42·Fs·Qts^−0.96. Port length comes from the Helmholtz condition with end correction: L = S·c² / (4π²·Fb²·Vb) − 1.46·r. Fb generally lands between 30 and 50 Hz, which is also where a domestic room's axial modes are densest — the box decides how deep the response goes, the room decides whether you hear it evenly once it gets there.
FAQ
Sealed or ported for my driver?
If you enter Qes, the tool shows EBP = Fs/Qes: under ~50 favors sealed, over ~100 favors ported, between works either way. High-Qts drivers (>0.5) generally want sealed or very large ported boxes.
Why does my port need to be so long?
Low tuning + small box + wide port = long port, and the trade is fixed rather than negotiable: length is proportional to port area, so narrowing the port to make it fit speeds the air up in the same proportion. The port velocity calculator walks that curve in both directions and says whether the diameter that stays quiet still fits inside the box. The other ways out are a passive radiator or a larger box.
How accurate is the response preview?
The sealed curve is the exact second-order response for your Qtc. The ported curve is an idealized 4th-order alignment sketch — real ported response depends on losses and the exact alignment, so treat it as a shape guide, not a measurement.