Transmission Line Subwoofer Box Calculator

Use this transmission line subwoofer box calculator to size line length, cross-section, stuffing, and folded volume before you cut.

📦 Transmission Line Presets

🔧 Line Geometry Inputs

Start with a draft line, then var the calculator estimate usable net volume and line length after driver, brace, and buffer allowances.

Line alignment

Straight TL boxes show Qtc and F3. Folded TL boxes calculate line length.

Line profile

Rectangle is the usual sub box footprint. Triangle works well for wedge trunks. Custom uses direct face area.

Internal length (in)

Longer length increases the face area.

Internal width (in)

Second face length for the footprint.

Custom line area (in²)

Enter a direct area when the face is irregular.

Line depth

Pick a quick depth or switch to a custom stack height.

Target line volume (ft³)

The calculator will estimate usable net volume for port math.

Driver size

Uses a typical displacement and recommended volume band for that driver class.

Driver displacement (ft³)

Includes cone, motor, and basket volume.

Brace displacement (ft³)

Volume lost to bracing and reinforcement.

Overage buffer

Build a little extra room for padding, leaks, and final trim.

Material type

Used for fit and shell weight estimates.

Net line volume

0.00

ft³ / L

Scale factor

1.00x

to hit target net

Recommended outer size

0.0 x 0.0 x 0.0

in / cm

Line Length

0.00

line length or straight TL check

0.00

Target net

ft³ / L

0.00

Gross box

ft³ / L

0.0

Shell weight

kg / lb

0.0

Line Length

TL build

📈 Reference Tables

Driver class guide

Driver	Straight TL net	Folded TL net	Disp.
8 in class	0.45-0.90	0.75-1.25	0.03 ft3
10 in class	0.75-1.40	1.10-1.80	0.05 ft3
12 in class	1.00-2.00	1.80-3.50	0.08 ft3
15 in class	1.80-3.50	3.00-5.80	0.13 ft3
18 in class	2.80-5.00	5.00-9.00	0.20 ft3

Line tuning guide

Alignment	Tune	Volume band	Use
Straight TL	N/A	0.45-2.00	Tight bass
Folded TL	33-35 Hz	1.10-3.50	Balanced
Low-tune	28-32 Hz	1.80-5.80	Deeper low end
SPL TL	36-40 Hz	2.50-9.00	Upper-bass hit

Fill material guide

Material	Thick.	Density	Note
15 mm MDF	15 mm	740 kg/m3	Light MDF
19 mm MDF	19 mm	760 kg/m3	Common build
12 mm birch ply	12 mm	620 kg/m3	Thin shell
18 mm birch ply	18 mm	650 kg/m3	Strong all-round
18 mm Baltic birch	18 mm	680 kg/m3	Premium panel
19 mm marine ply	19 mm	660 kg/m3	Moisture ready
12 mm HDF	12 mm	880 kg/m3	Very stiff
15 mm composite	15 mm	720 kg/m3	Stable board

Line profile guide

Driver	Area	Tune	Note
8 in class	12-16 in2	34-38 Hz	Small cabin
10 in class	16-22 in2	32-36 Hz	Tight daily
12 in class	22-30 in2	30-34 Hz	Most builds
15 in class	32-42 in2	28-32 Hz	Large trunk
18 in class	44-60 in2	26-30 Hz	Big output

💡 Transmission Line Build Tips

Tip: Net line volume comes after driver, brace, and port losses. Base the target on that final airspace.

Tip: TL designs need enough line area for airflow, or the tune will rise and compression will climb.

A transmission line subwoofer enclosure are a specific type of speaker box that use a long path. Many people attempt build transmission line subwoofer enclosures because transmission line subwoofer enclosures can create a smooth bass response. However, a transmission line subwoofer enclosure can also produce poor sound if the transmission line isnt sized correct.

A transmission line subwoofer enclosure allows sound wave to travel through a path, or transmission line. The length of the transmission line determine the frequency of bass the subwoofer enclosure will produce. If the transmission line is too short, the enclosure will produce upper bass frequency but will not produce low bass frequencies.

How to Build a Transmission Line Subwoofer Box

If the transmission line is too long, the transmission line will lose energy due to friction between the sound waves and the interior of the transmission line. As a result, the transmission line subwoofer enclosure will lose efficiency. The size of the driver in the subwoofer enclosure should match the volume of the transmission line subwoofer enclosure.

An 8-inch driver work best in a small transmission line subwoofer enclosure. A 12-inch or an 15-inch driver require a larger transmission line subwoofer enclosure. This is because the more larger the driver, the more area is required in the transmission line for the driver to move the air in and out of the enclosure.

If the air move too quickly in the transmission line, it will create air turbulence. Air turbulence can reduce the sound output of the transmission line subwoofer enclosure. By adding stuffing to the transmission line, such as polyfill or denim, the stuffing will absorb the middle frequencies of sound create by the subwoofer.

Additionally, by adding stuffing to the transmission line, it will act as if the transmission line is 20 to 30 percent longer then the actual length of the transmission line. The materials out of which a person builds a transmission line subwoofer enclosure are also important for the performance of the subwoofer enclosure. One of the most common material used to build the enclosure is birch plywood.

This is done because birch plywood resist warping. Another common material for building transmission line subwoofer enclosures is Medium Density Fiberboard (MDF). A person should use brace inside the transmission line subwoofer enclosure to prevent the panels of the enclosure from vibrating.

However, every time a person add a brace to the enclosure, that brace takes up some of the volume of the enclosure. In addition to the volume that the braces take up, a person must also account for the volume that the driver of the subwoofer take up inside the enclosure. To calculate the net volume of the transmission line subwoofer enclosure, a person must subtract the volume of the driver, the volume of the braces, and the volume of the stuffing from the gross volume of the enclosure.

The design of the transmission line will determine the type of bass that is emit from the transmission line subwoofer enclosure. If a person want tight bass sounds from their subwoofer, they should use a straight transmission line. If, however, a person want deeper bass sounds from their subwoofer, they should use a folded transmission line.

Additionally, depending on what type of bass a person wants to hear from their subwoofer, they will need to adjust the length of the transmission line. For example, someone who wants bass between 30 and 35 Hz will need a different length of transmission line than someone who want bass between 36 and 40 Hz. Finally, a person must also decide whether the transmission line will have a rectangular or triangular shape.

Finally, a person must avoid certain mistake when building the transmission line subwoofer enclosure. Using too much stuffing inside of the transmission line will reduce the output of the subwoofer enclosure. Another mistake that a person should avoid is not adding enough bracing to the enclosure.

Without the bracing, the panels of the enclosure will flex and create resonance within the enclosure. Finally, another mistake that a person must avoid is failing to test the subwoofer enclosure with an impedance sweep. An impedance sweep will allow a person to determine the frequency peak of the subwoofer.

Knowing these frequencies will allow a person to adjust the stuffing or the design of the transmission line subwoofer enclosure to increase its overall performance.