What this calculator does
A speaker driver is designed for an enclosure of a particular internal volume, and it is the internal figure that matters acoustically. Panel thickness is easy to overlook, and on a modest box built from 18 millimetre material it removes a surprising share of the volume, because the thickness comes off both sides of all three dimensions.
This calculator takes the external dimensions and the panel thickness, works out the gross internal volume, and subtracts an allowance for the space the driver and any bracing occupy inside the box.
The formula
Subtract twice the panel thickness from each external dimension, multiply the three results for the gross internal volume, and convert from cubic millimetres to litres. Subtracting the driver and bracing displacement gives the net volume the design should be checked against.
| Term | Meaning |
|---|---|
| Gross internal volume | The space inside the panels, before anything mounted inside is accounted for. |
| Displacement | The volume taken up inside the box by the driver basket, magnet, bracing and any port, which reduces the air volume the driver actually sees. |
| Net internal volume | Gross volume less displacement: the figure to compare against the driver specification. |
The inputs explained
| Field | What to enter |
|---|---|
| External width (mm) | External width of the box in millimetres. |
| External height (mm) | External height in millimetres. |
| External depth (mm) | External depth in millimetres. |
| Panel thickness (mm) | Panel thickness in millimetres. It is subtracted twice from each dimension, once for each side. |
| Driver and bracing displacement (L) | Combined displacement of the driver, bracing and any port, in litres. Driver displacement is often given on its specification sheet. |
When to use it
Checking a box against a driver specification
Drivers are specified for a net internal volume, so a box designed from external dimensions needs converting before the two can be compared.
Working back from a target volume
Adjusting the external dimensions until the net figure matches the target is the usual way to size an enclosure around a driver.
Comparing panel materials
Moving between 15 and 25 millimetre material changes the internal volume noticeably at a fixed external size, which matters when the outside dimensions are constrained.
Worked examples
Every figure in the tables below is produced by this page’s own calculator at build time, so the numbers and the tool always agree. Select any row to load that scenario.
How much volume does panel thickness cost?
The same external box built from a range of panel thicknesses.
| Panel thickness | Net internal volume | Gross internal volume |
|---|---|---|
| 12 mm | 27.14 L | 28.64 L |
| 15 mm | 25.47 L | 26.97 L |
| 18 mm | 23.87 L | 25.37 L |
| 25 mm | 20.38 L | 21.88 L |
Questions
Why subtract the thickness twice per dimension?
Because there is a panel on each side. A 300 millimetre wide box made from 18 millimetre material has 264 millimetres of internal width, not 282.
What displacement should I allow?
Driver displacement is usually on the specification sheet, often between half a litre and a couple of litres depending on size. Bracing and a port add more, and the port volume in particular can be significant on a vented design.
Does this work for a ported enclosure?
It gives the internal volume, which is the starting point for either type. A ported design also needs the port tuning worked out, and the port itself displaces volume that has to be included in the displacement figure.
Why do drivers specify a volume at all?
The air sealed inside the box acts as a spring against the cone, so the volume changes how the driver behaves at low frequencies. Too small a box raises the resonance and thins the bass; too large loses control of the cone.
For a plain volume of a rectangular space, see the cubic metre calculator. For the crossover point between drivers, see the low pass filter calculator.