Bass reflex calculator
Pick a driver, set net volume and tuning frequency – the calculator shows the frequency response, the −3 dB point and the port length you need (including end correction). It uses Thiele-Small parameters as a coupled mass-spring system with leakage and port losses.
Try it free in the app
How to design a vented box
A common starting point is the volume where a sealed box would have a Qtc around 0.55 – usually the most linear alignment. Then adjust the tuning frequency fb so that the response reaches low without a hump. Tuning too high creates a bump, too low gives a sloppy bass.
Port length and diameter
Port length follows from volume, cross-section and tuning (Helmholtz resonator). Very small ports cause chuffing at high levels – the app therefore checks port air velocity.
Frequently asked questions
How do I calculate the port length of a bass reflex box?
From the Helmholtz resonator: L = c²·S / (4π²·fb²·V) − end correction (about 1.46 × radius for two free ends). The calculator above does that for you.
Which tuning frequency is right?
It depends on the driver. Rule of thumb for drivers with Qts 0.3–0.4: fb about 1.0–1.4 × Fs. The calculator shows immediately whether the response stays flat.
How accurate is the result?
It is a simplified model (half space, ideal box, no stuffing or cabinet modes). The boxlab app calculates more precisely and can optimize automatically.