4th-Order Bandpass Enclosure Planner

Supported configuration. One driver separates a sealed rear chamber from a vented front chamber. This is commonly called a fourth-order single-reflex bandpass enclosure.

Volume datums. Rear volume is gross internal airspace, so the entered driver displacement is deducted there. Front volume must already be net free air after port, bracing and any hardware displacement; the calculator does not deduct those again.

What is calculated. Front-port tuning uses the shared UNSW-sourced Helmholtz model f = c/(2pi)sqrt(A/(VLeff)).

Worked example. With 20 L rear gross, 2 L driver displacement, 30 L front net, and one 10 x 30 cm round port at 20°C with no end correction, the front Helmholtz estimate is about 51.03 Hz.

The result above uses these exact values. This snapshot is included when the page is printed so the output can be checked against the original measurements.

Units

Litres / cm

Front port

Round

Rear sealed chamber gross volume

20

Front vented chamber net volume

30

Driver displacement in rear chamber

2

Round port diameter

10

Slot width

20

Slot height

4

Physical port length

30

Identical ports

1

Air temperature (°C)

20

Primary result: Rear chamber net: 18.00 L.

• Measure from the datum or reference edge described by this tool, and do not mix inside, outside and centerline dimensions.
• Keep inputs in the displayed units and preserve more precision than the final cutting or purchasing tolerance requires.
• When the result is close to a limit, verify it with a test piece, field measurement, manufacturer drawing or qualified project professional.

This does not design or predict the bandpass response, efficiency, excursion, group delay, cone loading, leakage, losses, port resonance or power handling. Chamber ratios are not universal design rules. Use measured Thiele-Small data and a loudspeaker simulation package, then prototype and measure. The port model has no end correction in this compact planner.