Measuring
Inside, outside and net
Everything acoustic runs on net volume: the air left inside once the panels, driver, port and bracing have taken their share. Measure the box you have (or plan) outside, enter the board thickness, and list what goes inside it.
The example is 386 mm × 586 mm × 330 mm outside in 18 mm board, which is 56.6 L inside. With a 3 L driver and the 100 mm port from the port calculator (3.6 L), it holds 50.0 L net.
Designing
From the volume you need
Switch to Depth when the width and height are fixed (by the driver, a shelf or a car boot) and the calculator finds the depth that gives your net volume. Box size builds a whole box from the volume in golden-ratio, cube or low-and-wide proportions.
Either way the drawing is to scale, the outside sizes are dimensioned, and the cut list is ready to copy.
Questions
Box volume questions
How do I calculate the volume of a speaker box?
Multiply the inside width, height and depth, then take away what sits inside. The example box is 386 mm × 586 mm × 330 mm outside in 18 mm board, so 350 mm × 550 mm × 294 mm inside: 56.6 L. Less a 3 L driver and a 3.6 L port, that leaves 50.0 L net.
How much volume does a subwoofer take up?
It depends on the driver: small woofers displace well under a litre, large subwoofers several litres. The data sheet usually lists it as driver displacement or volume displaced. Measure the magnet and basket if it is missing, and err on the large side.
Does panel thickness change the volume?
Yes. Each panel takes its thickness off both sides of the inside, so an 18 mm box loses 36 mm in every direction compared with its outside size. Enter outside dimensions with the real board thickness and the calculator works out the inside for you.
What is the difference between gross and net volume?
Gross volume is all the air inside the panels. Net volume is what is left after the driver, port, bracing and anything else take their share, and it is the volume a box calculator or simulator needs. A design that asks for 50 L means 50 L net.
What are the best proportions for a speaker box?
Unequal sides spread the box's internal resonances apart, and the golden ratio (about 0.62 : 1 : 1.62) is the usual choice. For 50 L net that is roughly 368 mm wide, 596 mm tall and 228 mm deep inside. A cube is the worst case for resonances, though bracing and lining matter more than small changes in shape.
How is the cut list worked out?
For a plain butt-jointed box: the front and back are the full outside width and height, the sides fit between them at the inside depth and full height, and the top and bottom fit inside all four. Allow for the saw kerf, and for a double-thickness front if you add one.