An enclosure is part of the electroacoustic system, not just a wooden box. Work from outside dimensions to usable volume and learn what enclosure simulations can and cannot promise.
Learning outcome: Build a volume budget, compare alignments and plan validation measurements.
External measurements include the panels. For a plain rectangular closed cabinet, subtract twice the wall thickness from each dimension. Convert cubic millimetres to litres.
Outside 260 × 400 × 280 mm, panels 18 mm: inside 224 × 364 × 244 mm, or 19.89 L gross internal space.
What inside width results from 300 mm outside and 15 mm panels?
270 mm. Subtract one panel on each side.
Drivers, braces, ducts and separate electronics compartments displace space. Net acoustic volume is internal space minus the occupied parts. Damping material may influence apparent compliance, but does not replace a volume budget.
19.89 L internal minus 1.30 L drivers and braces and 0.70 L duct and fixtures yields 17.89 L net.
What if you add a sealed 2 L internal compartment?
Subtract another 2 L from the acoustically connected space.
Trapped air behaves like a spring. Smaller box volume generally raises the system resonance; larger boxes approach the driver’s free-air behaviour. T/S parameters enable a first approximation.
Ideal small-signal estimate: Qtc = Qts · √(1 + Vas/Vb). With Qts = 0.40, Vas = 20 L and Vb = 20 L, Qtc ≈ 0.57.
What usually happens with much smaller Vb?
Qtc and resonance increase, which alone does not define perceived bass quality.
The air mass and friction in a port create a resonant system. Cross-sectional area, effective length and enclosure volume set Helmholtz tuning. Small ports may chuff; oversized ducts may become impractically long or develop resonances.
A 40 Hz port tuning does not automatically imply a 40 Hz -3 dB point. The driver and alignment decide.
Why check air velocity and cone travel?
Fast airflow can become audible, and cone excursion can rise steeply below tuning.
A passive radiator has no powered voice coil. Pressure inside the cabinet moves its diaphragm. Moving mass is central to tuning, and its excursion capability must be checked.
Adding mass changes tuning. The correct mass depends on the active driver, box volume and the PR’s own mechanics.
Why is area alone insufficient?
Moving mass, suspension and excursion also matter.
Joints should be airtight except for intended vents. Panels need appropriate stiffness; nothing should rattle. Validate the alignment with impedance and acoustic measurements rather than trusting a simulated plot alone.
Checklist: record net volume, measure duct length, inspect leaks, take an impedance sweep, compare near-field and listening-position results.
Which measurement reveals tuning especially well?
A suitable impedance measurement, alongside acoustic sweeps.
Plain rectangular cabinet with uniform panel thickness. Excludes unusual internal geometry.
These are simplified educational models. Calculators do not replace datasheets or measurements. Your progress remains in this browser.
