What it does
A woofer on its own makes little bass: the sound from the back of the cone cancels the front. Putting it in a box stops that, and the box’s air then acts as an extra spring. This designer uses the driver’s Thiele-Small parameters to predict the low-frequency response of two kinds of box side by side:
- a sealed box (closed box): a smooth second-order roll-off of 12 dB per octave, described by the system resonance fc and total Qtc;
- a ported box (vented, bass reflex): a tuned port extends the bass lower, then falls at 24 dB per octave below the tuning frequency fb.
It gives each box’s f3 (where the response is 3 dB down), whether there is a bump, the port length for your port size with end corrections, a rough worst-case port air speed, the cone’s displacement volume and the driver’s sensitivity from Qes. It can also size a sealed box for a chosen Qtc.
How to use it
- Copy Fs, Qts and Vas from the driver’s data sheet. Qes, Sd and Xmax are optional extras.
- For the sealed box, type a net volume, or type a target Qtc and press Size the box for this Qtc.
- For the ported box, type the net volume, the tuning frequency fb, the losses QL (7 if unsure), and the port diameter and number of ports.
- Compare the curves. Download the response as CSV, or the report as an image, PDF or Word file.
Worked example
The made-up example driver has Fs 35 Hz, Qts 0.38, Vas 60 L and Qes 0.42, with a 220 cm² cone and 6 mm of Xmax. In a 30 L sealed box the page shows fc 60.6 Hz and Qtc 0.658, and f3 65.6 Hz: no bump, a smooth roll-off. In a 50 L ported box tuned to 35 Hz with QL 7, f3 falls to 37.2 Hz with no bump. The port, one 7.5 cm tube, must be 161.4 mm long, and its worst-case air speed is 6.57 m/s, well below the 17 m/s where ports start to chuff. The driver’s sensitivity works out at 89.9 dB for 1 W at 1 m (η0 = 0.598 %). The driver alone in a wall would reach only 79.4 Hz.
Type 0.707 as the target Qtc and press the button: the sealed box becomes 24.37 L with f3 65.2 Hz. Retune the port to 40 Hz and a +0.6 dB bump appears above the tuning.
Tips
- Drivers with Qts below about 0.4 usually suit ported boxes; above about 0.5, sealed boxes.
- A room adds a lot of bass below about 50 Hz in a small room, so a sealed box that measures “too early” in this model often sounds deep at home.
- Leave room for the driver, the port tube and bracing: they take volume away from the net figure.
FAQ
Why does my port come out “too short”? The end corrections alone already tune the box lower than you want; use a narrower port or a smaller box.
Why is the curve different from my measurement? The model is free-field and low-frequency only: no baffle step, room, voice-coil inductance or cone break-up, and the parameters on data sheets vary from driver to driver. Measure your own driver for the best result. For study and planning, not a substitute for professional judgement.
Where the numbers come from
- Thiele, A. N. (1961) and Small, R. H. (1972, 1973), closed-box and vented-box loudspeaker systems, J. Audio Eng. Soc.: the 2nd-order closed-box and 4th-order vented-box transfer functions with box losses QL, used as formulas.
- Port length from the Helmholtz resonance with end corrections 0.8216 and 0.6133 times the radius (flanged and free ends); reference efficiency 4π²·fs³·Vas/(c³·Qes) and 112.1 + 10·lg η0 dB at 1 W, 1 m.
- Checked against an independent NumPy model of the acoustic equivalent circuit (driver, box compliance, port mass, leak) with SciPy root finding for f3, and against the Butterworth B4 alignment. The example driver is made up.
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Last updated 2026-11-09.