Boxes · Box type
BOX TYPEONE CHAMBERPORTVented
How it works#
The air in the port is a small mass; the air in the box is a spring. Together they resonate at the tuning frequency, Fb, the way a bottle hums when you blow across it. Around Fb the port moves most of the air and the cone barely moves; well above it the port hardly matters and the box acts much like a sealed one.
Below Fb the port and the cone work against each other and the output falls fast, close to 24 dB per octave: the example loses 22.8 dB from 20 Hz down to 10 Hz, twice the slope of a sealed box. The cone also loses the port's support there, which is the thing to watch.
What the tuning does#
The same 55 L box and 10 cm port, tuned from 28 Hz to 40 Hz. Only the port length changes.
SPL
dB
- 28 HzF3 29.6 Hz · Port 48.3 cm
- 32 Hz, the exampleF3 30.3 Hz · Port 35.6 cm
- 36 HzF3 32.6 Hz · Port 26.8 cm
- 40 HzF3 35.9 Hz · Port 20.6 cm
- Tune lower and the response sags into a long slope: F3 only moves from 30.3 Hz to 29.6 Hz, but the port grows from 35.6 cm to 48.3 cm.
- Tune higher and the response humps above the tuning: 3.1 dB over the level at 100 Hz at 40 Hz, against 0.5 dB for the example. F3 rises to 35.9 Hz and the port shrinks to 20.6 cm.
- A bigger box or a narrower port shortens the port for the same tuning; a wider port lengthens it. Ports and end correction has the details.
Excursion: low at the tuning, high below it#
Excursion
mm
- 32 Hz tuning at 200 WLeast 2.5 mm at 32.4 Hz · At 20 Hz 12.5 mm
At 200 W the cone moves least near the tuning, 2.5 mm at 32.4 Hz, because the port is doing the work. Below it the cone is unloaded and its travel climbs past the 13.5 mm Xmax below 18.7 Hz. If you do not need output below the tuning, a High-pass filter just under it keeps the cone in check.
The port has its own limit: at 200 W its air speed peaks at 28.4 m/s near 30.1 Hz, above the default 17 m/s warning line. This flags a risk of noise; the actual onset depends on the port’s shape and ends. The Port Velocity tile shows it, and Fix a noisy port shows what to change.
What you set#
Choose Vented in Type at the top of the enclosure panel. The vented fields are:
| Field | What it sets |
|---|---|
| Volume | The net internal volume: the air in the box, not counting the ports, the driver or bracing. |
| Tuning | The box’s tuning frequency, Fb, from 15 to 1,200 Hz. |
| Port length | The physical length of each port for that tuning. Tuning and port length are a pair: set one and the other follows. The one you set last is held when you change the port’s size: with the tuning held, a wider port gets longer; with the length held, the tuning moves instead. A warning beside it means the port is too narrow for the tuning: make it wider or add ports. |
| Port › Shape | A round port, a slot, or a custom Cross-sectional area. |
| Number of ports | From 1 to 10 identical ports, each as long as Port length. |
| Diameter, or Width and Height | The size of each port: a diameter for a round port, width and height for a slot. |
| Advanced | End correction and Port model, under Port. See Ports and end correction. |
| Results | Cross-sectional area (all ports together), Port volume, Gross volume (net volume plus ports and anything under Box volume adjustments) and 1st port resonance. |
When to use it#
Good for#
- Deeper bass from the same box. With the same Dayton Audio DCS165-4 in 10 L, the vented box reaches F3 35.9 Hz and the sealed one 77.5 Hz (one driver, four boxes).
- Little cone travel around the tuning, where the port does the work: more level for the same excursion.
- Drivers with a high EBP (Fs divided by Qes). When you add an enclosure, 00 Simulator suggests Vented for an EBP above 75; the Ciare 12.00SW’s is 88.
Watch for#
- The cone below the tuning, where it has no support: use a High-pass filter and read the Excursion tile at full power.
- Port noise: keep the air speed under the 17 m/s line at the power you plan to use, or make the port bigger.
- Port length: a low tuning or a wide port makes a long port. In a small box it may not fit; the comparison needs a 43.7 cm port in 10 L, where a passive radiator is the usual answer.
Try it#
The vented example at 100 W in the app’s own tiles, as the tuning drops to 28 Hz and a second port goes in.
To keep going, open the vented example in the simulator: the Ciare 12.00SW in 55 L tuned to 32 Hz, as on this page. Open it at 200 W to see the limits.
Tiles to watch#
Ask your AI#
Ask your AI
Using 00 Simulator, for the vented box in my open workspace, try the tuning 4 Hz lower and 4 Hz higher without saving. Tell me each option’s F3, port length and peak port air speed at its set power, and which one you would build.
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