Subwoofer tools.
ENCLOSURES & PORTS / THE TOOLKIT

Port length.

Calculate an effective and physical port length from net volume and target tuning. Slot-port end corrections are estimates, not exact predictions.

01 / THE INPUTS
Enclosure design
Hz
Port geometry
End corrections
Fit check

Optional. The real layout also needs clearance around the port ends.

Air & conventions
m/s
kg/m3
02 / THE RESULTS
Physical port length
447.1mm
Effective acoustic length
520.1mm
Total internal port area
78.54cm2
Combined end correction
73.0mm
Duct air volume, excluding material
3.51L

Read this result with its assumptions.

Physical length is per identical port. Net enclosure volume must remain available after internal duct displacement.

Sound speed: 343.00 m/s. End corrections are low-frequency starting approximations.

Units & assumptions

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03 / UNDER THE HOOD

The math, in plain sight.

Find a starting length. Then verify the tuning.

The Helmholtz starting point

At sufficiently low frequencies, the air in the vent behaves approximately as an acoustic mass. For N identical ports, use their combined internal area and the same effective length per port.

SΣ=NS1,fb=c2πSΣVbℓeffS_\Sigma=NS_1,\qquad f_b=\frac c{2\pi}\sqrt{\frac{S_\Sigma}{V_b\ell_{\mathrm{eff}}}}
ℓeff=c2SΣ(2πfb)2Vb\ell_{\mathrm{eff}}=\frac{c^2S_\Sigma}{(2\pi f_b)^2V_b}

Round ports and end correction

The last expression uses one unflanged and one substantially flanged end. Flares, nearby walls, and placement change the effective end conditions.

a=d2,S1=πa2,ℓeff=ℓphysical+Δinner+Δoutera=\frac d2,\quad S_1=\pi a^2,\quad\ell_{\mathrm{eff}}=\ell_{\mathrm{physical}}+\Delta_{\mathrm{inner}}+\Delta_{\mathrm{outer}}
Δunflanged≈0.61a,Δflanged≈0.85a\Delta_{\mathrm{unflanged}}\approx0.61a,\qquad\Delta_{\mathrm{flanged}}\approx0.85a
ℓphysical≈c2Nπa2(2πfb)2Vb−1.46a\ell_{\mathrm{physical}}\approx\frac{c^2N\pi a^2}{(2\pi f_b)^2V_b}-1.46a

Slot-port starting estimates

A slot uses its rectangular area in the tuning equation. The equal-area radius does not give an exact slot end correction. Aspect ratio, adjacent walls, and flares matter. Any slot result here is a starting estimate.

S1=wh,aeq=whπS_1=wh,\qquad a_{\mathrm{eq}}=\sqrt{\frac{wh}{\pi}}

For 50 L net at 30 Hz with one 100 mm round port and a 1.46a correction, the starting physical length is about 447 mm. A negative length means this simple geometry cannot meet the target. Include the internal duct displacement when establishing the final net volume.