Cabinet Tuning Basics: Box, Port and Fb

Cabinet tuning basics start with one resonance. In a reflex box the enclosed air is a spring and the air in the port is a mass; together they form a Helmholtz resonator at the tuning frequency, Fb. Box volume, port area and port length set it, and the driver's T/S parameters decide which combination works.

What happens around Fb

• At Fb the port supplies most of the output and cone excursion is at its minimum.
• Above Fb the cone radiates directly and the port contributes less.
• Below Fb the port unloads the cone. Excursion rises steeply and the driver can pass Xmax with far less power than it would in a sealed box.

A reflex box is a 4th-order high-pass system (24 dB/octave ultimate roll-off) against 2nd-order (12 dB/octave) for a sealed box, and it carries more group delay near tuning.

The formula

Fb = (c / 2π) × √(Sp / (Vb × Leff))

where c is the speed of sound (343 m/s), Sp the port area, Vb the net box volume and Leff the port's effective length. Rearranged for length: Leff = Sp × c² / (4π² × Fb² × Vb).

Worked example: 200 L net, 200 cm² of port, Fb = 35 Hz gives Leff = 0.02 × 343² / (4π² × 35² × 0.2) = 0.243 m. The physical port is shorter than Leff because air beyond each end moves with it (end correction, roughly 0.73 × port diameter for a round port with one flanged end). For a round port of this area, that brings the cut length to about 13 cm.

The three variables

• Port length: longer tunes lower. Fb scales with 1/√Leff, so halving Fb takes four times the length.
• Port area: at a fixed tuning, length scales directly with area. Double the area, double the length. An undersized port produces wind noise ("chuffing") at high level.
• Box volume: chosen from the driver's Vas, Qts and Fs for the alignment you want. With the same port, a larger box tunes lower; for the same tuning, it needs a shorter port.

A published reference point

B&C give a recommended reflex enclosure for their 18TBW100 (Fs 35 Hz, Qts 0.39, Vas 175 L): 200 L tuned to 32 Hz. That is the kind of figure a maker's model produces; your own driver's parameters will give different numbers.

To model a Wavemark WM1850/8, enter Fs 42.9 Hz, Qts 0.644, Vas 120.9 L, Sd 1244 cm² and Xmax 12 mm, then check excursion at the power you plan to use (rated 1800 W AES).

High-pass below tuning

Because the cone unloads below Fb, set a high-pass filter at or just below the tuning frequency. It is the most effective protection a reflex sub has against over-excursion. See Xmax and excursion and limiter settings for subs.

Model before you cut

WinISD (reflex) and Hornresp (horns and scoops) are free and take T/S parameters directly. Start with Thiele/Small parameters, or browse our speaker designs.

Sources: B&C Speakers published data for the 18TBW100 (8 Ω), checked October 2026; Wikipedia: Bass reflex; Wavemark WM1850/8 spec sheet. Helmholtz resonator relation: L. L. Beranek, Acoustics. Calculated values rounded.