Speaker Port Tuning Frequency Calculator (Bass-Reflex)｜Fb from Box Volume, Port Size, Temperature
Compute the bass-reflex tuning frequency Fb from enclosure volume, port inner diameter, physical port length, and air temperature using the Helmholtz resonance formula. Port area, effective length, sound speed, and A/V ratio shown alongside, with a 1.463·r end correction.
💡 About this tool
A bass-reflex (vented) enclosure uses the air mass inside a port acting as a spring against the box compliance. Near one frequency the port air moves in phase with the front of the cone and reinforces low-end output. That centre point is the tuning frequency Fb, set by port area, effective length, and box volume.
If you build DIY speakers or subwoofer boxes, the awkward part is predicting how a couple of centimetres of port length moves Fb. Working the square root by hand for every trial dimension gets old fast, and a wrong end correction throws the result off by a few hertz.
Punch in volume, port diameter, port length, and temperature and the tool returns Fb so you can nudge dimensions toward a target. It uses the effective length, not the raw physical length, applying the 1.463·r end correction typical of a port with one flush flange and one free end — the detail that keeps the bench number close to what you measure.
🧐 Frequently Asked Questions
Q. Which volume number do I enter? A. Use the net internal volume: the raw internal volume minus the displacement of the driver, bracing, and stuffing, expressed in litres.
Q. What exactly do I measure for port length? A. Enter the physical length of the tube that acts as the port. The calculator adds the 1.463·r end correction to get the effective length Leff, so the physical length is enough.
Q. Where should I set Fb relative to the driver? A. As a rule of thumb keep Fb at or above the driver's free-air resonance Fs. Tuning well below Fs lets cone excursion run high below the port frequency.
Q. Why does changing temperature move Fb? A. Sound speed c depends on temperature, computed as c = 331.3·√(1 + T/273.15). Between 15°C and 25°C, Fb shifts by a few hertz.
Q. What is the A/V ratio telling me? A. It is the ratio of port area to box volume — a quick check that the port is not too small. A small port area raises air velocity and makes chuffing (port turbulence noise) more likely.
📚 Why the end correction matters
The "effective length" the Helmholtz formula wants is longer than the tube you cut. Air just outside each open end moves along with the air column, so the port behaves as if it were slightly longer. The end correction accounts for that extra length.
The correction depends on how each end terminates. Two free ends give roughly 1.22·r, two flush-flanged ends about 1.70·r, and the common case of one flanged end plus one free end about 1.463·r (for a circular port of radius r). This calculator uses that one-flanged, one-free value, which matches the usual build: a port mounted flush in the baffle with the tube projecting inside. Skip the end correction and you over-estimate Fb; including it in the effective length is what brings the prediction in line with the measured tuning.