Dual Reflex Bandpass Enclosure Calculator

Size a sixth-order bandpass speaker box with two tuned chambers, round port lengths, passband span, chamber ratios, and port-speed checks.

🎵Descriptive dual-reflex presets

🔧Driver, chamber, and port inputs

Unit system Unit changes convert the chamber, port, Vas, Sd, and Xmax fields.

Box loss factor QL Used for the damping note and alignment check.

Driver Fs (Hz) Free-air resonance from the driver data sheet.

Driver Qts Dual-reflex alignments usually prefer moderate Qts.

Driver Vas (L) Equivalent compliance volume.

Cone area Sd (cm²) Used with Xmax for the port velocity proxy.

Linear Xmax (mm) One-way excursion estimate for peak air movement.

Apparent volume gain from damping (%) Applied to compliance checks; port math uses net physical volume.

Rear chamber net volume (L) Back chamber behind the cone, after driver and brace displacement.

Front chamber net volume (L) Front chamber feeding the listening side of the bandpass box.

Rear chamber low tuning FL (Hz) Lower Helmholtz tuning, usually near the low passband edge.

Front chamber high tuning FH (Hz) Upper Helmholtz tuning for the forward chamber.

Rear port inside diameter (cm) Round port diameter. For slots, use equivalent round area.

Front port inside diameter (cm) The high chamber often needs more area to control velocity.

Rear port count Multiple identical round ports share the total air flow.

Front port count Use whole numbers for matched vents.

This calculator uses round-port Helmholtz equations and alignment ratios for early design. Final acoustic response still depends on driver nonlinearities, leaks, wall losses, and port end details.

Tuning span

34-78 Hz

Center 51.5 Hz, 1.20 oct

Rear port length

26.1 cm

10.3 in, one port

Front port length

11.5 cm

4.5 in, one port

Total net enclosure

62.0 L

2.19 ft³ before port displacement

📊Calculated design snapshot

2.29x

Tuning ratio FH / FL

1.48

Rear Vas / Vb

2.35

Front Vas / Vb

15 m/s

Peak vent speed proxy

📐Dual-reflex alignment reference

Alignment goal	FH / FL range	Chamber balance	Typical use
Wide musical bandwidth	2.2 to 2.8	Larger rear, moderate front	Home theater, studio sub
Compact punch alignment	1.8 to 2.3	Smaller chambers, higher tunings	Small drivers, practice systems
Maximum narrow output	1.4 to 1.9	Tightly controlled front volume	SPL burp, limited band program
Instrument reinforcement	2.0 to 2.5	Moderate rear loading	Bass guitar, kick drum support

🎚Port and tuning comparison grid

Design style	Port count strategy	Strength	Watch item
Single rear, single front round port	One vent per chamber	Simple build and easy trimming	Velocity may rise on small boxes
Dual front round ports	One rear, two front	More high-band area	Longer front tubes for same tuning
Large slot equivalent	Convert slot area to round diameter	Fits shallow baffles	End correction changes with walls
Flared high chamber vents	Use larger effective diameter	Lower turbulence near FH	Flare takes internal volume

📝Common sixth-order project sizes

Project	Driver range	Net chamber range	Useful passband
Nearfield studio sub	8 to 10 in	18 to 55 L total	32 to 90 Hz
Car cabin SPL enclosure	10 to 15 in	45 to 140 L total	38 to 105 Hz
PA kick support cabinet	12 to 15 in	70 to 180 L total	50 to 130 Hz
Deep sub reflex rig	15 to 18 in	150 to 360 L total	24 to 75 Hz

💡Design notes

Use net chamber volume. Subtract driver basket, bracing, port tubes, handles, and any divider thickness before entering rear and front chamber volumes. The Helmholtz equation assumes the air volume available to the port.

Check port practicality early. If a calculated vent is shorter than one diameter or longer than the chamber depth, change diameter, count, chamber volume, or tuning before committing to panel cuts.

A sixth-order bandpass enclosure are made up of two separate chambers. The speaker driver is place in between these two chamber. Each of these chambers can be tune to a different frequency.

Because each chamber is tuned to a different frequency, the enclosure act as a filter that focuses the energy of the speaker driver into a specific range of frequency. Standard port box allow for a wide range of frequencies to emanate from the speaker, but the bandpass enclosure limits the frequencies to a specific window. The relationship between the rear and front chamber is the most important in a sixth-order bandpass enclosure.

How a Two-Chamber Bandpass Enclosure Works

The rear chamber is use to provide support for the low-frequency sound created by the speaker driver. The front chamber shape the sound that emanate from the speaker to ensure that the frequencies is within the desired range. To design the enclosure, the designer must calculate the tuning frequency of both the rear and front chamber.

The Helmholtz equation can be used to calculate these frequencies. By entering the specifications of the speaker driver into the calculator, you can calculate the required length of the box ports. The tuning ratio of a sixth-order bandpass enclosure is the difference between the low tuning frequency and the high tuning frequency of the enclosure.