Understand what the reference calculates
A Helmholtz resonator relates air volume, neck area and effective neck length. The reference uses sound speed 343 m/s, one circular internal diameter and the selected net box volume. It solves effective length for the requested tuning frequency.
Physical length subtracts a configurable end correction expressed in diameters. The default is an illustrative assumption, not a measured property of your installation. Flares, a nearby wall, a baffle, a bend and the shape of the opening change the effective behavior.
Do not interpret a length as a full vented design
The tool does not compute a vented loudspeaker frequency response or determine air velocity, turbulence, compression, excursion or power limits. Both plotted curves remain sealed-box models. The port reference is a separate calculation to record and compare, not an endorsement to add that port.
A nonpositive computed physical length is rejected as an unusable reference. A straight port longer than the internal depth is flagged. A shorter port is not automatically clear of the driver or adequately spaced from a wall; no placement has been designed.
Reconcile volume after choosing real parts
The occupied-volume allowance must include any physical port assembly alongside the driver and braces. The reference shows the internal air-column volume but cannot know the external tube wall, flare or mounting shape. Measure the final assembly and revise the allowance.
Changing the allowance changes the cabinet depth while retaining the chosen net volume. Save the final assumptions with the project and verify the actual acoustic result after assembly. This preserves the distinction between a calculated first model and a measured loudspeaker.