Driver size: what the inches mean

An "eight inch" driver is not eight inches across anything in particular. The nominal size is a trade label covering the whole frame, and different makers round it differently: an eight inch woofer has a frame nearer 220 mm and a moving cone nearer 165 mm.

It is useful for filtering a list and for knowing whether a driver will fit a hole. It is not what the physics depends on.

What the design actually uses

Sd, the effective cone area, is the piston that moves the air. Two drivers of the same nominal size can differ by 15% in Sd depending on how wide the surround is and how much of it counts.

Vas, the equivalent compliance volume, is what sets the box size. It depends on the suspension as much as the cone, so a stiff eight inch driver can want a smaller box than a floppy six inch one. Size does not tell you box size.

Xmax, how far the cone travels. A small driver with long travel can move as much air as a large one with little.

What moves air is Sd × Xmax, not diameter. That is the number to compare when choosing between drivers of different sizes.

What size does tend to bring

The correlations are real but loose, and every one of them has exceptions on the shelf:

• Larger drivers usually reach lower, because a bigger cone on a softer suspension carries more mass, and mass lowers f_s.
• Larger drivers usually shift more air, so they reach a given level with less excursion and less distortion.
• Larger drivers usually want a bigger box, because V_{as} tends to rise with cone area.
• Larger drivers give up the top end. A twelve inch cone begins to misbehave well before a five inch one does, which is why big drivers are crossed over low.

Choosing

Start from the output and the extension you need and work back to Sd × Xmax, then check V_{as} against the cabinet you can actually build. Use the size filter to narrow the list once you know roughly what class of driver you are after, not to decide what you need.

Two eight inch drivers and one twelve inch move about the same air. Which is better depends on the box you have room for and how high you need them to play.