BoxwrightWhy the tuning moves
The plan assumes a net volume, an end correction for each port end, and a box with no leaks. A port that ends near a wall acts longer than it is. Bracing and a driver with a deep motor take volume. Each of these moves the tuning by a few hertz, and a few hertz near 30 Hz is a lot.
The method
- Connect a tone generator to the amp. Set the gain low and the bass boost off.
- Play a slow sweep from about 20 Hz to 60 Hz, or step through single tones 1 Hz apart.
- Watch the cone, or rest a fingertip on the dust cap. At the box’s tuning the cone moves least, while the port blows hardest.
- Note that frequency. Check it twice, sweeping up and down.
Start quiet and turn up slowly. A long tone at high level can damage the driver and your hearing.
Why not the impedance dip
Many guides find the tuning from the dip between the two impedance peaks. In our check against Hornresp, that minimum sat about 1.2 Hz below the true tuning, because of box leakage. The point where the cone nearly stops is the tuning itself, so that is what we use.
Cut the port to fix it
A longer port tunes lower, a shorter one higher. The tuning goes with 1/√Leff, so to move from the measured Fm to the target Ft: new Leff = old Leff × (Fm/Ft)², and the length to cut is the new Leff minus the end correction. Cut a little at a time and check again.
| Port length | Change | Tuning |
|---|---|---|
| 12.8 in | −3 in | 34.9 Hz |
| 13.8 in | −2 in | 33.9 Hz |
| 14.8 in | −1 in | 32.9 Hz |
| 15.8 in | – | 32.0 Hz |
| 16.8 in | +1 in | 31.2 Hz |
| 17.8 in | +2 in | 30.4 Hz |
The table comes from the same port maths as the box calculator. Values for your own box: use the calculator with the length you built.
Sealed boxes
A sealed box has no tuning to find. The same slow sweep is still useful: listen for rattles, buzzing panels and air leaks around the driver and the terminal cup.
Sources
- Small, R. H., “Vented-Box Loudspeaker Systems” Parts I–IV, JAES 21 (1973)
- End correction: Rayleigh (flanged, 0.8488 r) and Levine & Schwinger 1948 (free, 0.6133 r)
Results are design estimates. Check the driver’s datasheet and your amplifier’s minimum load before you build.
Checked against the sources: October 27, 2026