Port length formula

Lv = (23562.5 × Rv² × Np) / (Fb² × Vb) − 1.463 × Rv

Rv - port radius (cm)
Np - number of ports
Fb - tuning frequency (Hz)
Vb - net box volume (litres)

Why port air velocity is not shown

Port air velocity cannot be calculated from enclosure volume, tuning frequency, port diameter, and port count alone. It also depends on the driver's effective cone area and excursion (or an equivalent acoustic-output model), plus the frequency and power being modeled. This calculator therefore reports only the port-length estimate and does not label a design as safe from chuffing.

Use the length as a starting point, then model the specific driver, amplifier power, signal, and port geometry in a complete loudspeaker-design tool. Port-end flares and clearance to the enclosure walls can also change real-world behavior, so a simple air-speed threshold would be misleading here.

Sealed vs. ported enclosure

The two fundamental subwoofer enclosure types have distinct acoustic characteristics:

Enclosure type	Characteristics	Best for
Sealed	Tight, accurate bass; gradual 12 dB/octave roll-off below resonance; requires more amplifier power for same SPL	Home theater, music with complex bass lines, limited trunk space
Ported (bass reflex)	Louder output at tuning frequency; more efficient; sharper 24 dB/octave roll-off below port tuning; larger box	SPL competition, genres with deep bass content, maximum output

Thiele/Small parameters explained

Fs (resonant frequency): the natural resonant frequency of the driver in free air. A lower Fs generally indicates a driver suited for deeper bass extension.
Qts (total Q factor): a dimensionless parameter that combines electrical and mechanical damping. Drivers with Qts < 0.4 typically work better in ported boxes; Qts > 0.7 generally favor sealed enclosures.
Vas (equivalent compliance volume): the volume of air with the same stiffness as the driver's suspension. A large Vas driver (highly compliant) needs a larger box.
Xmax (maximum linear excursion): the maximum distance the cone can travel while remaining linear. Higher Xmax allows more output at low frequencies without distortion.

Tuning frequency guidelines

The box tuning frequency (Fb) is the most important design choice for a ported enclosure. Different applications call for different tuning points:

Application	Typical Fb	Reason
Music subwoofer (home)	25–35 Hz	Extends deep bass for music accuracy
Home theater / LFE	20–25 Hz	Maximum extension for movie effects
Car audio (music)	35–45 Hz	Punchy output matching road noise floor
Car audio (SPL competition)	45–55 Hz	Maximum SPL in competition bandwidth

Tuning below the driver's Fs can cause excessive cone excursion and distortion. As a general rule, Fb should be within 10–15 Hz of the driver's Fs for best results.

Box material and bracing

MDF (Medium Density Fiberboard): the standard material for subwoofer enclosures. Dense and inert; resists resonance. Typical thickness: 18–25 mm (¾"–1").
Plywood: slightly lighter; wood grain provides some self-bracing. Less uniform than MDF; can delaminate with moisture. Used in car subwoofers where weight matters.
Internal bracing: cross-braces spanning the interior panels prevent panel resonance (panel vibration that adds audible coloration). Required for larger enclosures; place braces off-center to break up standing wave modes.