Ports and end correction

Where you find ports#

The controls are the same everywhere a port appears, apart from Port model, below. The examples on this page use the vented example.

Tuning and port length#

Tuning and Port length sit under the chamber’s volume, and both are always editable. Whichever you changed last is held, and the other is worked out from it:

• Change Tuning (the usual way) and the length follows. A different port size or count, chamber volume or end correction then changes the length, never the tuning. The example’s 10 cm port is 35.6 cm long for 32 Hz.
• Type a Port length and it is held instead: the tuning follows, and changing the port’s size moves the tuning. Cut the example’s port to 30 cm and the box tunes to 34.4 Hz.

The length comes from the port’s effective length: the length of air that resonates with the chamber at the tuning, less the end corrections.

effective length = port area × c² / ((2π × tuning)² × volume)
port length      = effective length − (inside + outside) × r

c = 343 m/s; port area is all the ports together; r is one port’s radius

Size, count and shape#

Every port below tunes the 55 L example to 32 Hz, so the SPL is the same for all of them. What changes is how long the port is, how much of the box it takes and how fast its air moves at 200 W.

• More port area means slower air and a longer port. Two 10 cm ports take the example from 28.4 m/s to 14.2 m/s, and each is 77.1 cm long; a 7.5 cm port is short but far over the 17 m/s warning line, at 50.5 m/s.
• The port’s air is not part of Volume, the net air the driver works against. It shows as Port volume under Results and adds to the Gross volume, the box you build: 67.1 L with the two ports against 57.8 L with one.
• Shape has three buttons: a circle for a round port (Diameter), a square for a slot (Width and Height) and a dashed square for a custom Cross-sectional area per port. A slot’s end correction uses the radius of a round port with the same area, so the 20 cm × 3.9 cm slot is the same 35.6 cm long as the 10 cm round port.
• Number of ports runs from 1 to 10. Diameter, Width and Height take 1 to 100,000 mm, and Tuning 15 to 1,200 Hz.

End correction#

Air just beyond each end of a port moves with the air inside it, so the port acts longer than it is. End correction adds that length back: a factor for each end, times the port’s radius. The more correction, the shorter the port you build for the same tuning.

Set it under Port › Advanced › End correction. The menu offers WinISD’s presets, each shown with its average factor, or Custom, which opens Inside and Outside fields from 0 to 3. The default is 0.6 at both ends, as in WinISD.

Because the tuning is held, the response does not move: only the length does, from 35.6 cm to 33.1 cm. A bandpass box’s two ports share one end correction: change it on either tab and both follow.

Port model and port resonance#

A port is also a pipe, open at both ends, with its own resonance where its length is half a wavelength. Results shows it as 1st port resonance, worked out from the physical length: 482 Hz for the example’s 35.6 cm port. If it sits too low the port lets through sound above the bass and colours the response, so keep it well above where the box plays.

Port model, under Advanced on a vented box or a horn’s ported rear chamber, decides how the solver treats the port. Lumped, the default, moves the port’s air as one mass, so the resonance is only a number in Results. Distributed treats the port as a duct that sound travels along, so the resonance shows in the graphs, and adds a Distributed duct resonance row worked out from the length with its end corrections.

With two 10 cm ports, each 77.1 cm long, the distributed model puts a peak of 10.2 dB over the lumped curve at 210 Hz and a dip of 8.4 dB just above, at 215 Hz, where real ports this long resonate too.

• Duct loss, in Np/m, damps the duct. It starts at 0.02 and goes up to 2. At the maximum the ports barely work: the two-port example’s F3 rises from 30.2 Hz to 75.5 Hz.
• Bandpass ports have no Port model control.

Air speed#

Fast air chuffs and whistles. The Port Velocity tile draws the peak air speed in the port against a warning line, 17 m/s by default; a sixth-order bandpass has a second tile for its rear port. At 200 W the example’s one port peaks at 28.4 m/s.

Try it#

Open the vented example at 200 W, open Port in the enclosure panel and change Number of ports to 2: the Port length grows from 35.6 cm to 77.1 cm, and the peak on the Port Velocity tile falls from 28.4 m/s to 14.2 m/s.

Ask your AI#

Related#