Gross vs net

Datasheets and box models care about the air volume the driver sees — usually called net volume. The empty rectangle you cut from the plans is gross internal volume. Anything that takes up space inside the box shrinks the air volume: the driver’s rear bulge, braces, ports, and crossover boards if they live inside. The rule of thumb from enclosure mistakes that kill DIY speakers: gross internal volume is not net. Here’s how to work it out.

Start from the target, then add displacements

  1. Read the recommended sealed or ported volume on the datasheet (or from your model). That number is almost always net. Or measure your own sample: measure Thiele-Small parameters. See how to read a driver datasheet and sealed vs ported.
  2. Estimate (or look up) how much volume the driver occupies behind the baffle. Maker drawings sometimes list a displacement; if not, treat the magnet/basket as a rough cylinder or truncated cone and convert to liters or cubic feet. Note which numbers are estimates.
  3. Add bracing volume. Rod Elliott (ESP) notes that bracing materials occupy space and need to be accounted for, and that builders often allow a little extra internal volume so late braces do not undersize the box.
  4. Add port volume for a vented box (the air inside the tube counts against the net chamber). End flares still displace wood and air — measure what you actually cut.
  5. Build to gross = net target + displacements. After glue-up, re-measure internal dimensions if you can; ESP notes that small volume errors usually matter less than the room does. Large misses still matter.

Bracing without wrecking the volume plan

ESP’s enclosure guidelines treat bracing as critical for panel stiffness and for pushing panel resonances higher. Prefer asymmetrical, off-center braces so sub-panels are different sizes. Window-frame braces work but are harder to place at odd angles. Aluminum U-channel can add stiffness while using little volume — only if you can glue it reliably. None of that replaces measuring what you installed.

Stuffing isn’t displacement

Stuffing isn’t solid displacement. Fibrous fill mainly damps sound inside the box and changes how the box behaves, so don’t subtract it like wood or a port. If your model assumed a fill amount, match it, then measure (REW basics; for sealed leaks, enclosure mistakes). How much to use is in How much stuffing to put in a speaker box.

Checklist

Panel thickness depends on the material: MDF vs plywood for speaker boxes.