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)

Port chuffing

Keep port air velocity below 17 m/s. Larger port diameter or additional ports reduce velocity. Flared port ends can also reduce noise at the port mouth.

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.