Sealed Speaker Box Volume Calculator

Sealed box volume from Thiele-Small parameters

A sealed (acoustic suspension) box adds its own "spring" — the trapped air — in parallel with the driver's own suspension. The smaller the box, the stiffer that added spring, which raises the system's total Q above the driver's free-air Q_ts. Solving that relationship for the box volume that hits a target system Q_tc gives: V_b = V_as / ((Q_tc/Q_ts)² − 1). It only has a valid (positive, finite) answer when Q_tc > Q_ts — a box can only raise Q, never lower it.

The resulting system resonance is F_c = F_s · (Q_tc/Q_ts). Q_tc = 0.707 (Butterworth) gives the textbook "maximally flat" alignment, where the −3 dB rolloff point F3 is very close to F_c itself — the standard target for accurate, unboomy bass. Lower Q_tc (0.5, Bessel) needs a bigger box and gives tighter, more controlled bass at the cost of extension; higher Q_tc (around 1.0) needs a smaller box and gives a small response bump just above F3 at the cost of transient accuracy and headroom (the driver moves more for the same SPL near resonance).

This covers the small-signal Thiele-Small model only — it ignores box losses (Q_l), driver Xmax/power handling, and stuffing/damping effects, all of which shift the real measured response slightly from the ideal curve.