Net and gross volume: two different boxes

Every simulator, this one included, works in net volume: the air the driver actually sees. It is the number that acts as a spring, so it is the number the physics needs.

It is not the size of the box you build.

What takes the difference

A finished cabinet has things inside it, and each of them takes air away:

• The driver. Its basket, magnet and motor stick into the box. A typical 8 inch woofer swallows 1 to 2 liters; a big subwoofer driver can take 4 or more. The figure is sometimes on the datasheet as displacement or Vd.
• The port. The tube occupies space that is no longer box air. Baffle knows your port geometry exactly, so it works this one out for you.
• Bracing. Shelf braces, window braces, corner battens. On a well-braced cabinet this is easily a liter or two.
• Stuffing, if you pack it densely, though a normal fill is close to negligible.

So:

gross volume = net volume + driver + port + bracing

What Baffle shows

The Net volume control is the air the model uses. Set it to the alignment you want.

Build this volume is the answer to "how big is the box", and it is the figure to divide into height × width × depth. It adds up everything that will be inside:

• your net volume,
• the port tubes, worked out exactly from your port geometry,
• the driver bodies, estimated from cone area,
• anything you enter under Bracing and extra.

The breakdown is printed under the figure so you can see which part is which.

About that driver estimate

It is an estimate and it is labelled as one. Driver displacement is not a Thiele-Small parameter and cannot be derived from one: it depends on basket depth and motor size, which no electrical measurement describes. What makes an estimate possible is that drivers are roughly the same shape as each other, so the body scales with cone area.

Calibrated against published cutout and depth figures it holds to within about ten per cent across 8, 10 and 12 inch woofers. Against a driver of unusual construction it will be further out: a neodymium motor is much smaller than the ferrite one it replaces, and a long-throw subwoofer with a stacked magnet is much bigger.

If you know your driver's real displacement, put the difference into Bracing and extra. If the alignment matters, measure it.

Beware the datasheet's Vd. Many datasheets list a "volume of displacement", and it is not this. It is Sd × Xmax, the air the cone moves. For a Dayton RS225-8 that figure is 150 cm³ while the driver's body takes roughly 950 cm³ out of the box. Using one for the other will put your box a liter out.

Getting the numbers

Driver displacement is on some datasheets. If it is not, the reliable way is to measure: seal the driver face down in a container of water, or fill the space behind the mounted driver with rice or polystyrene beads and measure what it took.

Bracing you can calculate from your cutting list: length × width × thickness for each piece.

Baffle already estimates the driver, so Bracing and extra only needs your bracing and anything else you are putting inside. A liter is a fair starting allowance for a well-braced cabinet.