How do I choose a port tuning frequency?

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.

What happens if the port is too small?

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.

Can I use a slot port instead of a round port?

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.

What is port resonance and how do I avoid it?

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.

AudioCalcs

Port Tuning Calculator — Bass Reflex Port Dimensions

Calculate port dimensions for a bass reflex speaker cabinet at a target tuning frequency.

Tuning Frequency

Target box tuning frequency Fb, typically 0.8–1.0 × the driver's Fs

Cabinet Volume

Internal (net) air volume of the cabinet in litres

Port Diameter

Internal diameter of the port tube

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

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Last updated: February 2026

All calculations are estimates based on standard formulas. Always verify results for critical applications.