Ported Box Calculator
Design a bass-reflex enclosure: set volume and tuning, get port dimensions, F3, port air speed, response and cone-excursion curves.
Inputs
How it works
A ported (bass-reflex) box adds a tuned air column, the port, to the enclosure. The air in the port and the air in the box behave like a Helmholtz resonator: a mass (port air) bouncing on a spring (box air). At the tuning frequency Fb the port output reinforces the cone, giving extra output but dropping off at 24 dB/octave below it.
Fb = (c / 2π) · √( Sv / (Vb · Leff) ) Leff = Lv + k · Dv (k ≈ 0.73 for one flared end) Lv = (c² · Sv) / (4π² · Fb² · Vb) − k · Dv
The response is computed with the full 4th-order Thiele/Small vented-box transfer function (lossless approximation). The excursion graph is just as important as the response: below Fb the port stops loading the cone and it flaps freely, an infrasonic filter is wise on ported systems.
Port air speed is estimated from peak cone displacement: v = 2π·Fb·Sd·Xmax / Sv. Beyond ~17 m/s (≈ Mach 0.05) ports start to chuff and compress; under 10 m/s is very safe. Fix it with a bigger port or more ports, which makes them longer.
The “auto-fill” button uses a quasi-third-order (QB3) rule of thumb: Vb ≈ 15·Qts^2.87·Vas and Fb ≈ 0.42·Fs·Qts^−0.96. Remember to add the volume taken up by the port itself to the gross size of the box.