The model
How port length and tuning relate
A vented box and its port make a Helmholtz resonator: the air in the port is a mass bouncing on the spring of the air in the box. The calculator uses the same relationship and end corrections as 00 Simulator, so a port you design here opens in the simulator with the same tuning.
Fb = c / 2π × √( S / (V × Leff) )
Leff = L + (kin + kout) × r
- Fb
- box tuning
- c
- speed of sound, 343 m/s
- S
- total port area, all ports together
- V
- net box volume
- L
- physical port length, the length you cut
- k
- end-correction factor at each end
- r
- radius of a round port with the area of one port
Worked through for the example: a 50 L box tuned to 30 Hz with one 100 mm round port needs an effective length of 520 mm. The end corrections supply 60 mm of that, so the duct itself is 460 mm long.
Design
Choosing a port size
The tuning fixes the ratio of port area to length, not the size. A bigger port needs a longer duct but moves its air more slowly, which keeps port noise down at high levels. A smaller port is shorter and easier to fit, but its air moves faster. The Other port sizes table shows both sides of that trade for your box.
How fast is too fast depends on the driver, the level and the flare of the port ends. The port velocity calculator runs a full simulation with your driver, power and high-pass filter, and the calculator above hands your box and port straight to it.
When the port will not fit
Enter the box’s inside depth and the drawing adds the back wall. A straight port should leave about one port diameter clear behind its inner end. If it will not fit, fold it with an elbow, run a slot along a wall, use a bigger box (a larger volume needs a shorter port for the same tuning) or accept a smaller port and faster air.
Questions
Port calculator questions
How long should a subwoofer port be?
It depends on the box’s net volume, the tuning and the port’s cross-section. A 50 L box tuned to 30 Hz with one 100 mm round port needs about 460 mm. With a 75 mm port the same box and tuning needs about 248 mm, and with a 125 mm port about 738 mm.
What is end correction?
The air just beyond each opening moves with the air inside the duct, so a port behaves as if it were longer than it is. The calculator adds a fraction of the port’s radius at each end: 0.600 by default, the value WinISD uses, 0.614 for a free end and 0.850 for an end flush with a panel. The drawing shows this extra air as dashed extensions.
How do I tune a port precisely?
Cut it a little long, measure the tuning and trim. For the 100 mm port in the 50 L example, cutting 10 mm off raises the tuning by about 0.29 Hz. The tuning of a finished box is the low point between the two peaks of its impedance curve.
Why does the calculator show a pipe resonance?
A port is also a pipe open at both ends, and it resonates where its length is half a wavelength: about 343 m/s divided by twice its length, 373 Hz for the example. A long port can put that resonance close to a subwoofer’s passband, so keep it well above your crossover.
Does a bigger port change the tuning?
Only through its length. For a given box and tuning, a larger port needs a longer duct and a smaller port a shorter one. The trade-off is air speed: for the same output, the air in a port moves at a speed inversely proportional to its area, so halving the area doubles the speed and makes port noise more likely.
Round port or slot port?
For the same area they tune the same box to nearly the same frequency, because the calculator gives a slot the end correction of a round port with the same area. Slots are easy to build into a cabinet and to run along a wall, but a slot that uses a box wall as one side can tune a little lower than predicted, so check the tuning after building.
How much volume does the port take up?
At least its inside area times its length, more with thick walls. The example port displaces 3.61 L, so to keep 50 L net the box needs about 53.6 L inside before you add the driver, bracing and any lining.