Subwoofer Port Length Calculator (Helmholtz)

It is — until the resonator flips it: near the tuning frequency, the Helmholtz resonance delays and inverts the backwave so the port's output arrives IN phase with the cone's front radiation, adding 3–5 dB of efficiency in the tuning octave. The price is paid below Fb: the port becomes a hole, front and back waves cancel, output dives at 24 dB/oct (vs sealed's gentle 12), and the now-unloaded cone flaps with full excursion — which is why ported subs need a subsonic filter and why sealed boxes still rule for the deepest extension-vs-size honesty.

Genre and driver, in that order: home-theater subs tune 18–25 Hz (LFE content lives there), music-first subs 28–35, SPL-competition car builds 40–50 (tuned at the scoring frequency), PA tops 45–60. The driver's Thiele-Small parameters then discipline the choice — classic alignments put Fb near the driver's Fs for flat response (QB3/SBB4 tables in the Cookbook), and tuning far above Fs trades extension for a one-note SPL hump. When in doubt: tune lower; the box forgives.

Air turbulence at the port mouth when velocity exceeds ~17–20 m/s — an audible 'whoosh' riding on bass notes, plus compression as the port chokes. Velocity scales with output and inversely with port AREA, so the rules: bigger subs and higher power need fatter ports (a 12-inch 500-W sub wants ~100 cm²+ of port area), flared ends raise the onset threshold ~30%, and multiple ports divide the load. The catch this calculator makes visible: doubling port area roughly QUADRUPLES required length for the same tuning — big, low-tuned, small-box ports get impractically long, which is why passive radiators exist (a port with no air, no length and no chuff, at the cost of money).

The impedance curve confesses: sweep the driver in-box (a $30 measurement jig or a DATS) and a ported box shows TWO impedance peaks with a valley between — the valley's minimum IS Fb. Off-target fixes are mercifully linear: port too long = tuned too low (cut it down — always build long and trim); too short = tuned high (extend with a coupler, or reduce area). Stuffing the box also lowers apparent tuning slightly (it fakes a bigger volume). Trust the impedance dip over any formula — including this one: real ports bend, flare and share walls, and the dip integrates all of it.