How port length and tuning relate
A vented enclosure and its port form a Helmholtz resonator. 00 Simulator solves the standard relationship Fb = c / (2π) × √(S / (V × Leff)), where S is the total port area, V is net box volume, and Leff is physical duct length plus its end corrections.
The calculator calls the same enclosure-model functions as the simulator; it does not carry a second page-specific Helmholtz implementation.
Worked example
A 50 litre net box tuned to 30 Hz with one 100 mm round port and the WinISD-default 0.600 correction at both ends needs a physical duct approximately 460.1 mm long. Its effective acoustic length is 520.1 mm and the first longitudinal resonance estimate is 373 Hz.
Net volume, displacement, and gross allowance
Net volume is the air space the acoustic model sees after subtracting the driver, port, bracing, and other solid objects. The gross-volume allowance shown above adds the calculated port displacement only. It is not a complete cabinet cut list.
What does end correction mean?
Air just outside each opening moves with the air inside the duct, making the port acoustically longer than its physical tube. The preset estimates that added length from an equivalent circular radius. A very wide wall-sharing slot can need a more specific model than this first release claims.
Why show the first port resonance?
A physical duct can support longitudinal standing waves above its intended tuning. The estimate is a useful warning, but placement, bends, damping, termination, and the real driver response affect what reaches the listener.
What this calculator cannot tell you
It does not predict port velocity, cone excursion, compression, audible chuffing, or acoustic response without a driver, input level, and filters. Use Check this port with your driver to carry the normalized geometry into 00 Simulator.
Continue with the Simulator guide, the vented enclosure reference, and the model limitations.