AudioCalcs
Port Tuning Calculator — Bass Reflex Port Dimensions
Calculate port dimensions for a bass reflex speaker cabinet at a target tuning frequency.
How We Calculate This
Port Length Formula
L = (c² × Sv) / (4π² × f² × Vb) − 1.46 × r
Where c = speed of sound (343 m/s), Sv = port area, f = tuning frequency, Vb = cabinet volume, r = port radius. The 1.46r term is the total end correction (equivalently 0.732 × diameter) — the sum of the flanged baffle end (0.85r) and the open inner end into the cabinet (0.61r). Use 1.70r (0.85 × D) if both ends are flanged.
Port Area
Area = π × (diameter / 2)²
Port Air Velocity
v = (2π × f × Xmax × Sd) / Sv
Peak air velocity through the port. Keep it below about 17 m/s (5% of the speed of sound) to avoid turbulence and audible chuffing; flared port ends raise this onset to roughly 30 m/s.
Frequently Asked Questions
A common starting point is 0.8-1.0 × Fs of the driver. Lower tuning extends bass but reduces maximum output. Higher tuning increases output around the tuning frequency but rolls off faster below it. Typical values: 30-45 Hz for subwoofers, 50-80 Hz for bass cabinets.
Air velocity in the port increases, causing turbulence and audible chuffing noise. Port diameter should be large enough to keep air velocity below 5% of the speed of sound (about 17 m/s). Larger ports require longer tubes for the same tuning frequency.
Yes, slot ports work the same way. Use the same cross-sectional area as a round port. Slot ports can be built into the cabinet walls, saving internal volume. They are common in home subwoofer designs.
Ports can resonate at harmonics of the tuning frequency, causing colouration. Flaring the port ends reduces turbulence and resonance. Keep port length under 1/4 wavelength of the tuning frequency to avoid pipe resonance issues.
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Last updated: February 2026
All calculations are estimates based on standard formulas. Always verify results for critical applications.