Design
Port velocity is a trade-off
Air speeds up when a small port has to carry a lot of air. More port area slows it, but the same tuning then needs a longer duct, which is harder to fit and brings the pipe’s own resonance down. Several ports share the flow, and share the length problem too.
The port length calculator shows that trade for any box, and hands its box and port to this page so you can add the driver.
The model
What the sweep shows
Each point is the peak air speed in the port at that frequency, from the full 00 Simulator vented-box model: the driver’s motor and suspension, the box, the port’s air mass and losses, and your filter. Power and filtering change the whole curve, not one number.
The comparison line is yours to move. Flare, the shape of the port ends, nearby walls, bends, the music and how far away you listen all change whether a given speed is audible, so treat the line as a way to compare designs, not a pass mark.
Questions
Port velocity questions
What is a safe port air speed?
There is no single safe number. A peak of 17 m/s, about 5% of the speed of sound, is a common rule of thumb for a plain duct, and flared ports can run faster before they are heard. The calculator draws that line so you can compare designs, and lets you move it.
How does power change the air speed?
Air speed rises with the square root of power: four times the power doubles it. The example peaks at 16.9 m/s with 130 W and 33.8 m/s with 520 W.
Why show cone excursion as well?
Because a port that is quiet enough does not mean the driver is safe. Below the tuning the box stops loading the cone. Without its high-pass, the example cone would reach 24.0 mm at 130 W against a 12 mm Xmax.
Why does the air speed peak below the box tuning?
Near the tuning the port does most of the work, and below it the cone and the port move against each other with little to hold them back, so the port air can move fastest there. In the example the peak sits at 26.3 Hz for a 30 Hz box. A high-pass filter below the tuning takes the top off it.
How do I lower the port velocity?
Give the air more area: a larger port or more ports. For the same tuning they need longer ducts, which the port calculator works out. Less power or a steeper high-pass also help. A different tuning or box moves the peak, and a passive radiator removes the port entirely.
Does this use the same model as the simulator?
Yes. It runs the 00 Simulator vented-box model over the whole sweep, in half space, and the button under the chart opens the same design in the simulator with its driver, power and filter.